Sorry, but I couldn't help it.

Being called a 'scholar' nowadays is regarded as 'name-calling' and being labeled derogatorily?

Anyways, without going into the headache calculations ... you were not short-changed.

A '+' (or even '-') prefix is only a measure of a lens' diopter, and NOT an indication of a fixed magnification capability, which as you found out, is dependent on various factors.

My suggestion is to only use the +1, +2 and so on designation as an indication of what filter you need, or for a reference.

If you need super-precise decimal point magnification calculations, that's different.