[transmetahuman]

1. In addition to multiplying the difference in form costs by 90%, reduce that difference by the percentage of the desired Max Duration. Also (or instead, if the AF costs less than the base form) put the Max Duration limitation on the 15 points base price.

2. Give the Alternate Form a Dependency, Fatigue damage only. RAW* says to make it Dependent on being in the other form for a pretty miniscule price break - I think Common. Personally I think it should be Rare for the same reason that abilities of the AF can be Always On even though they don't always affect the character - these kinds of things treat the form in isolation from the base form. Don't modify the base cost of AF or the AF's 90% at all (but that form will be cheaper because of the Dependency disadvantage).

*can't remember which book it's in, probably either Fantasy or Thaumatology