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You are asking about a bump gauge, sometimes called a "headspace gauge" incorrectly. Hornady and Sinclair both make very similar tools. For a bolt action rifle, the idea is to move the shoulder back .001-.003" from the point where you can just barely close the action on a case. For semiautos, the goal is about .003-.005". However, it's harder to find the point where a case just barely chambers. Disassembling the bolt probably is necessary, depending on the rifle.
If you are after more accuracy, then Hornady's OAL gauge is extremely helpful for finding the optimal overall length for a specific bullet in your particular rifle. IMHO finding the best OAL is an early step in making more accurate handloads.
The bushing resizing die is out of my league. At that point you need to pay attention to neck wall thickness, bullet diameter, concentricity etc. All important to maximize long-range accuracy. I simply don't shoot at that level.
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