The internal osc. of the 'F015 has 20% tolerance so, unless you are willing to calibrate every device, a crystal will do nicely :o)

I gave the following answer to a similar question on the Cygnal forum:
"Route SYSCLK to an I/O pin through the XBAR and measure its frequency. Based on this frequency you can then calculate the value for the Timer register(s). Obviously, this is not the easiest or cheapest solution ..."

Rob.