I know it all depends, But what is a general rule of tumb or a starting point on how many CFM to shoot for at your pond bottom depth.

It depends on the depth of your pond. livingwateraeration.com does a great job of summarizing it.
"The deeper an air diffuser is located, the more boiling action it will create and a larger area will be aerated. Therefore the deeper your pond the less CFM needed to aerate your pond. 1.5 CFM of air can effectively aerate a 1 Acre pond at a depth of 12' or deeper. The chart below can be used to determine the size of the compressor that you will need, it is based on a compressor producing 1.5 CFM of air." - See more at: http://www.livingwateraeration.com/poaeinpa.html#sthash.o0Mjq0Q2.dpuf

Bobby, it all depends on how efficient your diffusers are, and the shape of the pond, in addition to the depth.

Basically you need to bring the total pond volume of water up to the surface at least once every 24 hr.

Pond depth, diffuser lifting rates, cfm of the pump at depth of the diffuser(s) all figures into the equation. There is no simple answer, and without knowing those figures, it's impossible to calculate.

Diffuser lifting rates at different depths

thanks , I am getting a better understnading now.