You adjust the dilution depending on the temperature and desired development time. 68 degree equal 20° C, correct? Suggested development times for dil. B (1+31) can usually be found inside the film boxes and online. If you find these times to be too short, you just double the dilution to dil. H (1+63) and get the double time.
I usually use dil. B during winter when our kitchen is pretty cold (min 17° C) and for pushing. During the summer it can get about 25° C and development times get too short. I this case I just use dil. H.
If I shoot with a flash, I don't care about ISO and stand develop for one hour (turn tank upside down after 30min) in a dilution of 1+100.