Well, the calculations for rainfall over surface area could be used— if one could determine the total surface area of the worlds oceans. but like your Niagra Falls based calculation for the visual effect of 2,620 connected Falls output... to produce one inch raising.
Notionally, of course the very concept of melting ice caps raising water levels (but only in certain places... heh) is absurd. If they completely melted, both of them, and the ice quantity accurately figured— maybe 3, 4 inches and that would have to be everywhere— all shorelines.
“ if one could determine the total surface area of the worlds oceans.”
You cant use the surface area only to calculate it. Raising the ocean level changes the surface area itself