Hello Fullen,
No, you do not need the second FPERIOD() call.
The formula should be:
=E24/(((ADGET("gldates.END","P"&FPERIOD())-ADGET("gldates.START","P 1"))+1)*99)
This formula subtracts the ending date for the current period
last year (because you are using "P" before the period number - for the current year you'd use "C") from the starting date of last year's fiscal year.
(Presumably you have 99 rooms available each day, and E24 contains your total room revenue).