That's a very important letter that you have from CASA, I Fly. I'd like to get a copy.
I actually enjoyed calculating the climb gradients for my airplane - the good news was that mine complied with CAO 20.7.4.
I wonder what response I'd get if I followed that letter up asking for exact instructions on how to do a Cessna 150 AFM under the new rules. I bet that, even if I got a straight answer, it would be a lot different than the way it's being practiced.