How to calculate drain current that i will supply in HPA

Hi:

How to calculate drain current that i will supply in HPA ?

assuming the output power is 15W, Vdd=28V, the drain efficiency is 25%, and give 60% margin. Attachment is a part of the datasheet.
my calculate method is :

Pdc = 15W/25% = 60W
Idc = (60/28 ) / 60% = 3.6A

Am I right? If wrong,please give me some design guide

thanks

cntral