I would use 27500. It's a nice even number and likely makes up for the sloped walls. Either way you're close enough that it doesn't matter too much. Another thing to ponder is whether the depth is exactly 3' and 8'. 

I guess you didnt understand my question is the 3 ft measurement to mid skimmer or to the very top of the pool?
I'm just wondering... what bleach are you using? Could it possibly be some of last year's inventory that's been sitting out in the garden section?
Or did poolmath somehow shift to the default and use 10000 gallons as the pool size? Or is the stuff 8.25% bleach and it moved to 12.5%?
I had that problem about a month ago. I fixed that by putting the reagent bottle back in the test kit and pulling out the correct one! Boy, did I feel dumb!!! Not saying that's your problem.I just did the first test of the morning. It looks like my FC is down to a 2.5 and get this - I cant even get the CC test to complete. I've added 20 drops and still going, no difference in the shade of pink. I've done the CC test multiple times and keep getting that result.
How is that possible? Or what can I possibly be doing wrong with the test??? I add the 10 ml of water to test for the FC, add the 5 drops of the 003 solution and it goes to pink and then I start with the 0871 and keep going and going and going..
I did a CYA test for kicks and it's still registering virtually nothing. The test kit I have is the TF-100 - anything else in there I can test for that would be impacting these numbers? Talk about a roller coaster ride!