Fitting a guard screw is 101 GS stuff. Its a very, very, very common practice. This might help:
1/4-28 screw. 1/4" in diameter and 28 threads per inch. Easy enough. This is the most common one used by action makers and gun companies here in The States. Divide 1 by 28 and you get .0357". That will become relevant here in a second.
So, drop the bolt in your action. Put the front screw in first. Tighten it down. Now insert the bolt and GENTLY attempt to close. If the bolt hangs up, stick the appropriate tool in the guard screw and note its clock position. Now start backing out the screw while holding the action in the stock. At some point the bolt handle falls. When it does, note how many rotations your tool made when backing it out.
Multiply the rotation by the pitch (.0357") and add one half rev for clearance. That is how much you remove from your screw. Reset, cycle start and do the same on the rear. Tang screws with blind holes are a bit more work. Basically I just chip away at those till they work.
Good luck..