[MattKing]

If you use a #2 filter to start then the next filter is the next grade.

If you use no filter, then find you need to go up or down a grade, what filter do you pick, and what is the exposure change?

(If you know the answer to that - or have worked it out - great. But this used to bug me... I didn't really know how far the next step or what exposure it would take.)

Bill has it right.

Also, when you use the #2 filter for an exposure, and then decide that a #2.5 filter would be better, you will be able to use an exposure time with the #2.5 filter that is either the same or very close to the same as the exposure time you used with the #2 filter.

It also helps to systemize your process - if you have a workflow that always deals with contrast filtration, you will be much less likely to forget to use the filtration when you mean to do so.