A five set is not meant to be maximal effort. We do max efforts down in the 3-2-1 rep ranges. It'd be a mistake to jump on the bar today and do a true max set of 5 reps.
If you were to get more scientific about your weights today, you'd be best served by determining your 1 RM and then calculating a percentage (see below).
To do this correctly you have to prepare for it well in advance. For this, complete the warm-up as described above. From there, complete 3-5 sets of increasingly heavier weights. Begin with a 15 set at a light to moderate weight. Rest 5 full minutes between sets. Subtract 3-5 reps per set and progress the weight slowly from light to heavy. Once you've completed the warm-ups (it should take a solid 20-30 mins), try to determine your 1 RM over the next 2-4 sets. This shouldn't need to be said, but we'll say it anyway - USE STRICT FORM! There is no purpose in injuring yourself in preparation for exercise.
Once you've determined your 1 RM use 86% of that weight for your sets of 5 AHAPs today.
Somehow you can always spot the guys who Rx their lunges. (Scott)