Stone: shooting at slower than box speed is more exposure and will give you thicker negs, not thin, unless you're reducing development far, far too much. It's the opposite of pushing. Shooting it at lower speed means you can reduce development and therefore tame the contrast a bit. Gives you a longer tonal scale and less risk of unprintable negs.

Anyway, my typical Pan-F (EI25) development is 6:30 in Rodinal 1+50, rotary. What also looks good at EI25 is D76 1+3 (therefore no solvent action) for 14:00, traditional inversion development.

Definitely agree that you should shoot, develop and print a test roll or two beforehand to make sure you have the process at least approximately dialed-in.