500 EFI exhaust length required to prevent reversion?
 

I know there are a lot of variables including angles, but in general, how much of a run is required for the exhaust before water is introduced to prevent reversion on a 500 EFI?

I am assuming that there must be a formula for this.

No formula. It can also depend on how the boat sits in the water at idle. 500EFI's never had a problem with reversion and many had the shorter silent choice risers.

Now I'm really confused. I have been told that my Stainless Marine risers won't work on my new 500 EFI's. They are the shorter ones. 3" taller than standard.

The cam in the carbed 500HP's were prone to causing reversion. The 500efi cams have less overlap and did not have the issue. Your short risers may be a concern. You could have the inner pipe lengthened or have SM lengthen them.
I would just run it and then check for reversion by pulling the risers and inspecting for water and go from there.