"you have no idea where it will end up."
True, but with a history of retarded cam timing in the 70's, chain (or belt) stretch, low compression (often from pistons TDC being down in the hole) I seriously doubt you will wind up with valves hitting pistons or too high a cylinder pressure if you advanced say..., 4 degrees (as a max). You are probable just putting the cam back to some prior, less stringent smog year (and new belt) degree setting. I seriously doubt it will go out of the range of acceptable.
And in the end the setting will be "what works." That is the whole point of the adjustable sprocket. It is because you desire to deviate from the manufacture setting. That part is done by testing, not math. I wouldn't argue at all if you are building a race motor with tight piston to valve clearance, compression ratios that push the limit of cylinder pressure etc.. Though I'd bet that regardless of where an engine was set when built, not too shortly after it is run the timing set stretch and initial cam wear have it off anyway.
But here (on a stock 70's era 2.3), you loosen the bolts, you rotate the sprocket, you test drive. When it is to your liking (if it even gets there) then curiosity might factor into seeing what the number really is.