Don't feel bad. You applied common sense and while your numbers are probably not accurate, they would be safely on the conservative side. I would not have thought of adding the 20%, so well done there!
Old light twin aircraft like the Apache were not certificated to any proper performance standard that we would accept today. The second engine was more a perceived safety device than an actual one.
When you start your ATP studies you should get proper and representative flight manuals to work with. Then you will have real numbers.
Last edited by Mach E Avelli; 5th Apr 2014 at 08:03.