Mark,
Honestly it just becomes an very well-educated guess. You learn your lens and collodion mixture. I sometimes do a test plate before I start with customer tintypes, then go from there. Of course, exposures change somewhat throughout the day. Also, keep in mind that I hand-develop, so that gives more control with exposures.

Scott,
The question about time; collodion gels before putting into silver (@20 seconds). From the time pouring collodion to fix should be less than 10 minutes for entire process. Then of course is washing, drying, and varnishing.
J. Coffer's manual will make everything make more sense.

Ty Guillory
www.tystintypes.com
Mineola, TX