easily-remembered rules

Hi, mg. I, too, usually hyphenate by instinct, but I think the most basic rule is use a hyphen whenever needed for clarity. Another rule, if I remember correctly, is that you do use a hyphen with a two-word modifier unless the two words are usually read as one. IMO, whether "third floor" qualifies under that exception is a judgment call, but I probably would have hyphened it. I do know that a hyphen is NOT necessary with "ly" words - that's one rule that does stick in my mind (must have had an English teacher who drummed it into us). So, from one hyphenation queen to another, hyphen away! We'll find a use for all the ones other people leave out.