The temperature you use to temper is going to depend on the type of steel, and the intended use. Higher temperatures leave the steel not as hard but tougher, lower temps leave it harder and more liable to cracking/breaking under stress. Please note that 'stress' is a variable term; it can mean anything from hacking through a dry branch that feels hard as a rock to slicing thin layers off a piece of wood to slicing a roast where you might hit a bone.
As to type of steel, carbon adds wear-resistance, which is why good knives generally use high carbon. However, properly worked medium carbon can make fine blades, especially for big ones. Note that a medium carbon steel will need less heat to get it from fully-hardened to the working hardness you want, while high carbon will often need a bit higher temp to keep it from being brittle.
With O1 tool steel for a general-use knife, I'll start around 400F for the first heat. I don't have access to a Rockwell hardness tester, so I go by color and experience. If you've shined up the surface nicely before heating, when you take it out of the oven after 45-60 minutes, the blade will have a nice dark yellow/light bronze color, which seems to work out to about 50-60 Rockwell, just about what you want. If you decide to increase the temp, do it in 25 degree steps; up it 25, temper 45 minutes, let it cool, and then test it. If it's a little too hard, can up it another step. Please note, if you temper it softer than you wanted, you either live with it, or go back to the fire to harden it again and start over.
The basic test for edge hardness is very simple, and works quite well. I picked it up from an article by Wayne Goddard, and all you need is a piece of 1/4" brass rod and a vise. First, sharpen the knife as you normally would. In good light so you can see well, clamp the rod in the vise so one side is above the jaws of the vise. Take the sharpened blade and hold it at a shallow angle against the brass as if you were going to slice a layer off and push down gently, watching the edge where it contacts the rod. A properly hardened & tempered edge will flex visibly, and return to straight when you release the pressure. If it chips, it's too hard; if it flexes and stays bent, it's too soft. I've found it very accurate as a test.
Testing can get involved. The American Bladesmith Society tests blades for Journeyman and Master status harshly, sometimes to destruction. Heavy cutting, edge-holding ability, and the final is to clamp the point in a vise and flex it; if it cracks or breaks before flexing the right amount, it fails. What I generally do is a bit simpler. With big blades, 8" & up, I save up small(2" or so) limbs when pruning trees for the chopping test, which is chopping them into shorter pieces to make sure the blade will hold up, and stay sharp. For actual edge-holding, I take 1/4" manilla or sisal rope, set a board in a vise for a cutting surface, and start cutting rope. I try to make the cuts no more than 1/2" apart, and stop when it won't cut cleanly any more or I get tired, depending. I've had blades cut well over 100 pieces and still be useably sharp; they might not shave anymore, but they'll still cut cleanly.
One further test is for the smoothness of edge; take a magazine page or a piece of copy paper, hold it along one edge and see if the edge will slice it cleanly. When you finish sharpening this is a good check, because any rough spots will snag and tear the paper. After the cutting testing, this tells you if the edge is still fairly smooth, or if it picked up tiny nicks during the cutting.
One more thing; a new-made blade may not show it's best edge-holding right off. One of the things steel does, when it hits the critical temperature for hardening, is that it can lose carbon atoms into the atmosphere. So, depending on how long it was at heat, you may have a thin skin of decarburized steel on the piece(steel that is at a lower carbon content than that below that layer). This layer will not hold an edge as well, but as a time or two of sharpening wears past it, you get to the high-test stuff and it picks up performance. Usually the finish-grinding and polishing cleans it off, but if you sharpen and test before that, it's something to remember.
There's another step that some makers are using on blades, a sub-zero quench. Basically you take a hardened blade, clean it absolutely clean of all traces of oil or whatever, and lower it into a tank of liquid nitrogen. Leave it soaking long enough for the entire piece to have reached that temperature, then slowly remove it and hang it up to warm to room temp. After that, to the oven to temper normally. I've heard of using this both before and after tempering, but I haven't tried it myself. Reports indicate that it can make a serious improvement in the toughness/edge holding ability of some blades. Basically, when steel hits certain points, the structure of the metal changes. Freshly hardened steel contains a structure called 'martensite' cause by the structure freezing when quenched. The sub-zero quench is supposed to cause a more complete transformation to martensite than would otherwise happen. Or so I'm told. The problems with this are two: 1. liquid nitrogen doesn't last too long, even when kept in the proper container it evaporates and 2. the stuff is at about 250 below zero, and if you splash it on yourself, instant burn. It will literally kill tissue on contact, so if you decide to mess with the stuff BE CAREFUL!
That about covers it, at least for now. If you try making blades and doing your own heat-treatment, you'll be doing some experimenting to see what seems to work best, and expect to screw things up a few times; there's no way around it. And you'll keep learning as you go.