You should be able to judge most of the time what weight to use after your first set. lets say, for example, that on your first set of flat bench you usually push 245 for a set of 10, this time you put it up for 12. If you got those 2 extra reps pretty smooth then you know the weight you usually go with will be increased by 2 reps. Say that your shooting for 8 reps on your 2nd set, just add 10-15lbs to the weight you went with the week before.
Hope this helps a little. This is the fun part while on so dont get frustrated, have fun.
Machine