My understanding, and how I believe the spreadsheet is coded, is the following:

The level armor constant of 16635 for level 83 targets effects how much armor they have. This is already factored into the target's armor amount. Different levels of targets and their armor constants factor into the armor amount for target's of various levels.

The reason that the level 80 armor constant of 15232.5 is used in the calculation of the ArP cap is because it is believed that the attacker's level affects the maximum amount of armor that can be penetrated. Hence, the armor constant for our level is used to determine the ArP cap.

Thus, both our level and the target's level factor into how much of the armor can be penetratable.

Here is another way to look at it. Say if there are two targets with the same amount of armor, but one is level 80 and one is level 83. One would think that we would be able to penetrate an equal or lesser amount of that armor on the level 83 target than the level 80 target since it should be harder to kill. If the armor constant for the target was used in the ArP calculation, then we would actually be able to ignore more of the armor on the 83 target than the 80 target as shown, making it easier to kill instead of harder.

If target's level was used in ArP cap calculation:
- Level 80 target with 10K armor results in 8411 ArP cap
- Level 83 target with 10K armor results in 8878 ArP cap

With using the attackers level to determine the armor cap, we ignore the same amount of armor on a level 80 and level 83 target with the same base armor; however, since level 83 targets will tend on average to have higher amounts of armor than level 80 targets due to the higher armor constant, more of the armor on average level 83 targets will be unpenetratable.

Please ignore. Sorry.

Volley DPS values in the same spreadsheet are slightly misleading though, since a lot of the averaging of procs and talents we have in the sheet don't apply when we volley - trinkets other than greatness/death'sverdict don't proc while volleying, master tactician doesn't proc while volleying, replenishment (and I think thrill of the hunt too) don't proc while volleying. Gftt at least does proc though, so your pet's DPS isn't hurt.

Getting volley to show on the Calculations or shot rotation tab would be nice, but it would probably need to be based on not averaging AP/crit effects from most trinkets and procs (I've tried periodically interruping volleys when master tactician drops for instance and firing shots till MT procs again, it's unreliable enough to not be easily modelled in the sheet IMO). With that caveat though, a row showing your volley dps per target (DPS, not damage per tick, since haste affects volley dps without affecting tick value) or per N targets would be great for gearing decisions in situations you know you'll have to volley. I would go so far as to say the DPS number should be shown on the Gear tab itself, so you can easily see it change as you swap gear.

This brings up the issue though that I have no idea how exactly Volley damage and tick-interval is calculated. Has anyone worked out a formula for it?

The other day I decided to recheck my dps and fiddle around with the spreadsheet to see how certain talents would work for me. To my amazement it is now saying that dropping Arcane Shot from my rotation nets dps. The reason i say this is that I don't use an arp trinket, I don't gem arp and I have even specced into Arcane Shot. All this being said it still is telling me that Arcane Shot in the rotation is a dps loss. I looked over parses from some raid fights and its showing that Arcane hits for about 300-500 more on average than Steady Shot. Also the spec I'm checking out doesn't pick up IMP SS so I have no idea what is going on. Is there anything I'm overlooking or did Steady somehow miraculously beat out arcane shot at some gear level?