Richard Robinson wrote:

Correct me but didn't Imperial Ranma jump into this Suicide
Ranma for a few minutes, so he could have gotten an idea of
what happens. Enough to be able to estimate the most likely
result of a permanent jump.

He did.  Due to the shortness of the time, that visitation might only be
remembered subconsciously.  Combine that with a few notes on the subject in
Genma's notes, and it would coalesce into an escape plan.

Jurai Knight