55ga sump help

[tuck]

Hi Everyone

I finally got tired of looking at all the HOB stuff I have on my 55ga. I'm going to build myself a sump. I want to put my protein skimmer, 2 little fishes reactor with carbon and heater in it and have a refug. The actual building/assembly is not a problem but determining the size of sump and the size of pump to return water to the tank is cause for further research. I plan on using a CPR overflow (the small one, for up to 60ga). I believe it's rated at 300gph. I know the sump has to be large enough to hold the excess water from the tank in case of a power outage but exactly how do I figure that out? I would prefer not to use trial and error - that could get messy.

I'd appreciate any help, experience. Thanks