For rainwater calculations in the US, this is a great site. It also works for any area, but it can calculate monthly rainfall for you with a US zipcode:
http://rainharvestcalculator.com/Calculator
As for the chest freezer-to-fridge conversion, we have one of those and I would add one key thing most people don't mention... caulk all the seams inside the chest freezer. We used a silicone, I think it was one for aquariums, may have been for kitchens.
We've been using this setup for a fridge for three years now. Because it doesn't have a drain setup like a standing fridge does, they collect a lot of condensation at the bottom. We clean ours out every two weeks or so. The more things you store in there like vegetables or fruits - things that will transpire - the more moisture. This isn't a big issue to me, but it's good to know ahead of time. The chest fridge does what it needs to and uses very little electricity, and I actually lose (aka forget and let rot) less food in it versus our small upright fridge.