Wayne;
Best way to number LEN is through CMD 14. LEN = 5 digits XXYYY
XX = FP number
YY = Slot number
For an FP defined in programming as real (ie 00-03), the slots range from 000-127.
For an FP defined in programming as a virtual (ie 16-19), the slots range from 000-063.
CMD 10 is still valid, but NEC recommend using CMD 14 as it opens up the possibilities of Virtual FP's for IP Dterms etc...
Cheer.