Complications arise due to barrel length differences. Say the book shows 20" and you're shooting a 16". 25fps loss/inch is just a WAG. It could be more or less depending on the load.
That's what a chrono is for; but for rule of thumb guesstimates, I use 35 FPS change per inch of barrel change, but as we all know, book loads don't always yield the same results in the real world, even in the same barrel length as was used to determine the load.
MM
