I'm not sure I have the whole context, but *if* you're saying that you want to create another [foo], and you still have the first [foo] around, then how you match gauge is - empirically. take your best guess for a first gauge swatch, and change up or down from there as appropriate.

now, if you've sent or given the [foo] to someone else, that's much tougher. but if there's a suggested total pattern measurement (e.g. circumference, for a sweater)... gauge math could still provide an answer to the question.

I agree though that having more complete project notes would be helpful. and i too am terriblly lax about writing them down... if i forget what's in my head, it can be really tough to regroup.

good luck!