Of all the parts of the English language, the one FLG feels he has little to no grasp of, it's compound words. Are there any rules at all or is it one random jumble?
Why checkup, but follow-up?
Why drive-in, but sellout?
Why run-on and show-off, but takeoff?
None of it makes any sense. So, FLG's approach is pretty much just to wing it. Generally speaking, FLG includes the hyphen unless he happens to know the commonly accepted spelling omit it.
In fact, FLG objects to the idea that spelling a compound word with a hyphen should ever be considered incorrect. Take-off is just as clear as takeoff, maybe more so. Same for sell-out and check-up. Drivein, on the other hand, looks weird because the first word ends and second word begins with vowels. So, the opposite case, removing a hyphen is not optimal.
So, be liberal with your hyphens, and let's leave crazy-long-unhyphenated-words to the Germans.