The expected finished weight of the aircraft is a better method of determining what you will need. There is no "rule of thumb" anymore since the advent of 3D. It's all about thrust to weight ratio. How much thrust that a given size engine provides will determine what you can do. A thrust to weight ratio of 1.5 to 2-1 is great for 3D. A 3/4 to 1-1 thrust to weight ratio is great for warbird type flying. Then you come into planes with extremely light wing loadings and it all changes again, permitting smaller engines than either the 3D-aerobatic types or the warbirds. Sometimes it can get pretty confusing even for those with a lot of time in this stuff.