Don't use the Gallon per inch rule too literally....It depends on the Fish. Do some research on the web or ask here in regards to the type of fish you have or looking to get. (Also the Search function here is good)
For Example an Oscar needs a lot of filtration and the inch a gallon doesn't really apply due to its heavy Bio load.(messy eater, eats a ton and provides a lot of waste) It is always good to 2x the size tank you have. You really can't go wrong with too much. A lot of the gph
flow rating is without Filter media. So if you are looking to be covered go with the 70 gallon rated filter.
I am the extreme I have a Penguin 350 and a Aquaclear 70 on my 30 gallon. Ends up to be like 500 gph
, but I am still stocking..It is a African Cichlid tank.