The blog post I referred to in my previous post, contains a theory that when the ammonia is stated to be 5-10%, it is not because the manufacturer is sloppy when mixing it up, anything but (they want to save money, be efficient and consistent), but 5% refers to the amount of ammonia in the solution, and 10% refers to the weight ratio.
Regarding development issues. The blogger also states he performed a couple of inversions over the 15 minutes he let the Acros (in this case) stand. Letting it stand for so long seems a bit excessive, but works in his case most likely because at some certain point during the process, the fixer has overtaken the developer, so adding a few minutes may not harm the development much, but rather enhance the fixer's ability to do it's job.