Hi, there isnt a formula
the only thing you've to understand is: a bigger sensor means smaller dept of field.
a p&s has a small sensor (so: great DoF)
a dslr has a big sensor (so: small DoF)
I know how to explain it in Dutch, but my English is not very good, Ill give a try.
When using F/2.8 on a p&s you get a small DoF
When using F/2.8 on a dslr you get a smaller DoF because of the bigger sensor. you understand this?
When closing the aperture on a dslr to, let's say, F/5.6, you get a bigger DoF, BUT, not as big when using a compact when using F/5.6
So, I hope Im right, the aperture is almost the same (however: when my lumix is set to 2.8 is has a much smaller iris when I set my rokkor-w to f5.6, anyone could explain this me?) But, because of the bigger sensor, you have to close the aperture for getting the same effect.
This is however almost never a problem: many people buy dslrs because of getting a small DoF!