Anyway, it's funny how I didn't find that much about action at the nut. It's not menitoned in the guides I've read but it's very important.
This is an excellent point and very astute of you to pick up on. In my experience, very few instruments come from the factory with the nut action dialed in. It can be tricky to get right, because of course if you go just a hair too far you may need a new nut. Some people will build up a nut slot that's too deep with super glue and then file a new slot, but of course it's preferable to not get that far in the first place.
To really do it right, adjusting nut action requires tools that most people don't have (i.e., [Invalid or Expired Link Removed]), which may be one reason that a how-to-set-up-your-bass guide might omit it. There are tricks and shortcuts that might work, such as wrapping sandpaper around a string, or even using a string as a file/saw. (My recollection is that Rotosound steel roundwounds, for instance, feel a lot like saw blades under the fingers!) (I kid.) (Not really.) The other thing you definitely need is some sort of feeler gauge or [Invalid or Expired Link Removed] to measure the clearance of the string at the first fret. In this regard, given the clearance range you are looking for (I usually shoot for .010" on guitars and maybe .015" on basses) you can always use a length of guitar string of the appropriate thickness.
Finally, this is coming late to the thread, but, here's a simple process for setting action on a bass or guitar that requires no special tools or measurements. This is different than doing a full set-up and it sounds like you've already gotten beyond this, but I'll just put it here in case it's helpful to somebody:
Play up and down each string. Do they buzz unacceptably anywhere?
No:
Are you happy with how the bass plays?
Yes: Congratulations! You're done.
No: Lower the string height a bit and repeat.
Yes: If you are getting buzz
On the low frets by the nut, but not on the upper frets by the body: Add some more relief to the neck, and repeat.
On the high frets by the body but not on the low frets by the nut: Take some relief out of the neck, raise the string height a bit, and repeat.
Everywhere: Raise the string height a bit, and repeat.
Only in one specific place on the neck: Sorry, but you have a high or low fret, and you will need some sort of fret leveling or re-seating---have a tech or luthier check it out.
When using this procedure, as you get closer, you can make smaller and smaller adjustments. Eventually you will get to the point where it's as good as it can be. If your neck is not warped or twisted and your frets are in good shape, even a cheapo bass can be made to play remarkably well.
Learning to set up your own instrument can be intimidating but once you figure it out it's an invaluable skill to have. Among other things, you can often tell whether a poorly-playing instrument is really a dog, or just not set up properly, which can get you bargains on good-quality-but-bad-setup instruments that other people may overlook. I got a sweet deal not too long ago on a Thunderbird that played poorly in the store, but it just needed a truss rod adjustment. Other people probably played it, felt that the action was horrible, and passed. From knowing how set-ups work, I could look beyond the excessive neck relief and see the potential in the instrument.
Good luck!
