The AVR data is sparse here, they give a formula, but no numeric ranges.

If I look at the register mapping, I infer that just 4 bits are used for M,N, but that hardly seems right ?

ie if your PLL goes to 240Mhz, then /16 stops at 15Mhz, missing a whole range of crystals.

Is there any hard info, on the numeric ranges that the PLL can actually work with ?

The whole idea of a PLL, is to allow more choices from a given crystal, and on a 32 bit chip, allocating very small numbers to PLL ranges, does not make sense.

We need to cover 3 target frequencies, and with good precision.

I see the LPC17xx has a useful PLL spreadsheet, that takes numbers, and reality-checks them - is there a AVR32 PLL-Calc equivalent ?