you have to take into assumption some stuff...
For example you'll have to assume that the nuclei have constant density...
and you'll have to assume that their shape is given (to be spherical)...
I think all formulas are basically another way to calculate R= 1.2 A^(1/3) . Sure, you can replace A by the mass divided by the (average) mass of a nucleon, and get a formula like R= const* (mass/density)^(1/3), but that is nothing new.