My NEXUS card and passport both say First Middle Last. I have successfully recieved precheck with LAST/FIRSTMIDDLE, LAST/FIRSTMIDDLEMR, LAST/FIRSTMIDDLEINITIAL, LAST/FIRSTMR, LAST/FIRSTMIDDLEINITIALMR, and even once LAST/FIRSTMRS (I'm a male, it was a booking error).
I think the whole "exact match" hysteria is just hype - the database seems to be able to match my KTN with my name and birthdate without issues.