Water: Evaporation Rate Again

I've been having a really tough time getting a consistent post-boil volume on my all-grain batches. My evaporation rate is inconsistent batch to batch despite using the exact same boil time, process and equipment.

For example:

Evaporation test with water: 5.9 quarts in 90 minutes.
Batch #13: 4.1 - 4.2 quarts in 90 minutes, so I ended up with 13.75 quarts instead of 12.
Batch #14: 0.6 quarts in 15 minutes, which is about 3.6 quarts in 90 minutes.
Batch #15: 5.8 - 5.9 quarts in 97 minutes, which is 5.4 - 5.5 quarts in 90 minutes.

All done with the same Winware 5 gallon pot on the same burner of my electric stove, and even adjusted to the same location on the burner within a millimeter of error. Humidity is generally about 55% here, and the temp has been around 72 on every brew day.

The only thing I can think of that is different is boil volume. More water in the pot takes more energy to maintain a vigorous boil. Given the anemic BTUs generated by my stove, the more water there is the less vigorous the boil will be, and thus less evaporation. Let's follow this line of reasoning:

Evaporation test: 18 boil volume, 5.9 loss in 90 minutes (maybe longer, I didn't get a warning when the water hit boiling since my stupid thermometer has an auto-shutoff 'feature'). This also may have included cooling loss, though. Plugging into BeerSmith actually gives 20%/hour, 5.4 quart loss, 0.5 qt cooling loss. Doh! Real result is 18 to 5.4.

Batch #13: 18.5 boil volume, 4.15 loss in 90 minutes. This is an anomaly; it is way too low.

Batch #14: 8.2 boil volume, 3.6 loss in 90 minutes. You'd expect this to be higher, rather than lower, if the 'vigorous boil' hypothesis is correct.

Batch #15: 16.9 boil volume, 5.4 loss in 90 minutes. Consistent with the original evaporation test, but with a lower boil volume.

I'll keep taking samples until I get something that actually works. For my next batch, I'll try 17.5 quart boil volume and an assumption of 5 quarts evaporation in 90 minutes.

edit:
Batch #16: 17.75 boil volume (higher than expected due to less loss to grain than predicted by BeerSmith), 5.64 loss in 90 minutes. Will try 17.9 boil volume next time.

edit:
Batch #17: 17.88 boil volume, 5.85 loss in 90 minutes. Will try 18.2 boil volume next time.