How to write a naval-policy alternate history

What advice would you give someone writing an alternate history based on one nation's change in naval policy or doctrine?

Keep the initial change simple and historically close, research its consequences carefully, and decide on a firm end date.

Keep the initial policy or doctrinal change close to the historical situation and research it carefully. A simple divergence is easier to manage than an expanding set of unrelated changes. Set a sensible end date before the project grows into an indefinite timeline. The long-running Red Fleet project supplies the example of how later consequences can keep drawing a writer beyond the intended endpoint.