How about adding an additional syntax to the spell
so that you could cast a weaker featherweight?
cast feather weight at fnurt try 1
would for example attempt to cast a 1 hour featherweight.
Would be useful for merchants especially...
Knowledge of metamorphosis could be used to determine
what is the range that you are able to attempt.
With 0% mastery cant reduce time at all, with 100%
mastery you could have a minimum duration of 1 hour.