It's a 15 gallon sender, when the warning light comes on, it will take 9-10 gallons to "fill up".
I've read about cutting the vent tube here on the forum. Probably will do that if I don't have to replace the sender, or just use the 19 gallon pump if I do have to replace it.
I've done a couple of searches and it appears to be a fairly common problem, I just can't find any threads about how anyone fixed it, other than:
1. Just guess how much is in the tank and hope for the best. (Which is what I've been doing)
2. Carry extra gas.