Brian -- I do not believe the ASGCA has any "policy" regarding fees. It's clearly up to members. The percentage "calculation" is one way fees -- all fees of all consultants -- can sometimes be estimated when a project is somewhat standard in terms of its infrastructure and complexity. But, of course, not always. I usually get into trouble when I try and figure things this way. In fact, I recently said to a group on a remodel that I thought the fees -- GCA, engineer, irrigation designer, etc. -- would be "about 10% of what they would end up spending on the work". I was off a few percentage points as the work was very complicated and our dear engineer and irrigation consultant came in well above my guesses. This provate club was not impressed and I had to re-state my words "estimate...", "about...", etc. I am still alive, so it all worked out.
We do not figure fees based on a percentage, and I doubt many other GCAs or ASGCA members do either. I think this is a calculaion still regarded in engineering and some builoding architecture fields. I know several landscape architects and I don't think it is how they do business any longer.
I prefer, really, to just quote $1 million and slightly extra if we need to use our corporate jet.