Ideally, you want an 18" or 24" straightedge and some feeler gauges. Lay the straightedge from the 1st fret to however far it goes, and measure the clearance at the midway point. I don't know what it's supposed to be on a bass, but I gotta think it's in the .025" - .030" range. But, there's an easy way to find out - check a bass that's already set up the way you like it.
If you don't have a straightedge, you can use a string and a capo, or if you don't have a capo get someone to help you because this'll takes three hands. Fret the string at the first fret, then pull it taut and fret it at the 18th or 20th fret as well. Measure the clearance under the string about midway. Take notes, or adjust as needed.
Tightening the truss rod will gradually flatten the neck until you go too far and start to put some back bow in it. That's NFG. You don't want that. Loosening the truss rod will "relieve" it, increasing your string clearance.
It usually doesn't take much adjustment to bring it in. Think 1/4 turn or so. Also, it may move once you string it back up to tune, so you may have to do this twice.