Working with varying reps is too difficult to track. I'll give an example scenario on a better way.
Rather than benching 150 for 5 reps, then 6 reps, then 8 reps, then 10 reps, do this:
Bench 150 for 6 reps. Next time, bench 151 for 6 reps. Next time, bench 152 for 6 reps. Increment the weight, not the reps. If your weight increments are small enough (usually 2% or less), you won't even notice a difference. You are lifting more, though, and therefore you will get stronger. It may seem slow and odd but it makes perfect physical sense.
Say your goal is to add 20lb to your bench press. You can work the usual multivariable (weight and reps) process and be confused and unsure the entire time... or, you could simply work with tiny incremental weight changes, static rep amounts, and know for sure that you are stronger in week 7 than you were in week 3.
Just make sure to keep the increment low enough that you are not overtraining, and remember that it is not a race. It is a marathon. The difference between a 20lb gain in 10 weeks vs a 20lb gain in 14 weeks is negligible. The cost of injury, however, is great.