One of the most accurate ways to estimate your max at any given time is to do indicator sets. An indicator set is a heavy load where you will do 2 – 5 reps and cannot do another. The reason you do less than 6 is at six reps, due to time under tension, the energy systems begin to switch from ATP-PC to the lactate system and the variability is greater.
After you do your heavy indicator set for as many as possible for 2 – 5 reps, just use some simple math. Take the total number of reps and subtract one. Multiply what is left by 3%. Then multiply this number by the load used in the heavy indicator set. Add this number back to the load used in the set. For example:
5 reps at 225
5 – 1 = 4
4 x 3% = 12%
.12 x 225 = 27
27 = 225 = 252
This works very well and can give you an idea of where you are in your strength development during training without having to prepare to max or max.
What if I want to know what my max is on a lift in which I do not max? This is quite common, as many lifts are not tested, but an athlete wants to cycle this lift. Here are the percentages for a couple of lifts to train percentages off of without having to max on those lifts.
Incline is 80% of bench press
Snatch is 70% of the Clean
Squat is 80% of the Dead Lift
These are excellent rules of thumb to use in order to train with percentages without having to test several different lifts.