I agree with Rocketman. The file test is a good one, if the file slides across the steel then it is close to being as hard as the file. Probably Rockwell 60-62. For a screwdriver I think that is too hard, easy to break the tip and damage surface.
In making your own, getting the right steel is critical. To do the job right you should have an oven like Rocketman said. On small parts you can use a oxygen/acetelyne torch with the piece sitting on a scarp piece of steel on a fire brick and carefully heat the steel to cherry red (2500F) and quench in used motor oil (if it is oil hardening tool steel), clean carbon from piece by sanding and it should be a moltted greyish color. Re-heat to a light straw color (1250-1350F) and quench again. Try the file test and see if it "bites" into the steel, if ok then grind to fit screw.
Without an oven trying to heat treat long (4"-6") pieces you will warp them using a torch, you need a controlled heat and to bring the temperatuare up gradually. The torch does it in seconds and would warp a long piece.
Good luck in what you try, but the best way in my opinion is to buy cheap screwdrivers with some temper and grind them to fit.
