Bob Tait's book works well, I used it for CFPA.
If all else fails, ask your instructor for a few hours time, they may point you in the right track, fix any shortcomings you're having in your echo calculations, and would cost you less than doing a course.
Check out
Downwind.com.au, for my CFPA Notes. Just do a search for CFPA and it should come up. (I tried linking but it came up with the login page every time...
Login)