is there a rule of thumb ,that with ideal conditions. a certain amount of cfm added equals(x) amount of horsepower?

Are you talking about head flow? The rule of thumb for head flow is that cfm is 2 hp. So a head that flows 300 cfm should be capable of supporting 600 hp. This is barely a rule of thumb since it is only roughly correct but it gets you in the ballpark. The assumption is that the engine builder doesn't do anything stupid either upstream or downstream of the heads.

And that it is a V8, a 4cyl is 1 to 1.

2 hp per cfm is a very solid N/A build, but if by "ideal conditions" you're referring to a maximum effort N/A build, well past 2 hp per cfm is routinely observed. For example, a PROPERLY ported W8 at 400 cfm, with the required supporting cast, will push well past the 800 hp level without breaking a sweat. 2.4-2.5 is obtainable, but requires serious refinement in every aspect of the engine.

With so many variables in play to maximize a particular combination, peak head flow being part of the overall package, is not the end all number in determining maximum power output. Not all cylinder head flow is created equal.

.257xcfm@28hsig =hp of 1cylinder x # of cylinders. As stated anything done to increase VE will bring HP up. Formula only works with ACCURATE data.