Steve,

One good way to find out your true focal length is with plate solving. The focal length specs for most scopes are a rough approximation.

The effects of an AO or filters should be negligible (these should be designed to have zero power) but flatteners, reducers and barlows will certainly have a significant effect.

You'll find a formula for the CFZ and a calculator here:

http://www.wilmslowastro.com/software/formulae.htm
The CFZ is dependent on wavelength (which is why CCDAP gives you different values).

Cheers,

Rick.