Kalavo,
Just work it out for yourself from first principles and you will see what we are getting at.
When you recall that an approved CASA method of determining PSD for exam purposes was a piece of string cut to the scale length for safe endurance and 2 pins to stick in the appropriate airfields, I expect you will see what I mean.
Have since done so, ta. Trying to get back in to the books (knocked off all the CASA exams long ago, but no where near current) that was one this was missing from my notes and had me stumped late one night. Bit more sleep and it's a lot simpler than I first thought.