define multicycle path on specific instant

I would ALWAYS put a fifo across the domains for things like that,there is usually too much risk you'll catch it on a change at some point, so you'll get intermittant failures. But if you really have to do it, you can use the set_false_path command, with a similar setup to the multicycle path.

For your design, as far as I can tell:

set_multicycle_path -from [get_keepers {fpucorr:fpu1|*}] -to [get_registers {fpucorr:fpu1|*}] -setup 3

2 for hold because Ive always been of the understanding 1 less than the setup for hold was best.