Consider a subject, and estimate some dimension of it. For example, if your subject is US quarter, then you might say the diameter is an inch or so.

Next ask yourself, how big is that subject on the ground glass (GG)?

From the actual dimension of the subject and the GG dimension, deduce an approximate magnification (m). For example if the quarter is thesamesize on the GG, then m=1. If it is twice as big on the GG then m=2.

Next compute the bellows factor from this simple relation:

Bellows factor= (1+m)^2

Example: if you have the quarter at life-size on your GG, then the factor is (1+1)^2=2^2=4.

Multiply your exposure by this factor. So if your hand-metered exposure for a 1:1 shot is 1/25 sec, then your exposure corrected for bellows factor will be 1 sec.

That's it. For b&w film especially, that's all I've ever needed. For slide, I sometimes reach for a ruler or set a coin or piece of paper next to my subject to get a more precise measurement. But 99.44% of the time, the magnification is what I guessed by eye.

I think it's important to remember this basic approach because I always forget rulers and meters and whatever. It's really easy. Easier than computing reciprocity factor. I mean, people often just wing it on reciprocity corrections or dev times, but when it comes to bellows factor they schlep all manner of tools and tape measures and so forth. Pack less stuff. Keep it simple