First let me say that hand setting bearings is not that bad. When I type it out it's gonna sound long and drawn out, and it can be. Especially maybe the first time. It is a little bit fiddly, so you can expect it to take some iterations, and some time, but it's very simple steps, very approachable, and a lot less daunting if you know what the end goal is. Don't let this throw you off. In an application like this, they need to be good, plenty good even, but you don't need anywhere near theoretical perfection. The bearings won't ever see any load that's going to challenge them. And I'm going to throw some numbers at you, and while this is a "feel" thing in the end, these (rough) numbers give you some reasonable targets as to what's enough, too much, or too little during process.
If you're able, if the bearing manufacturer has a website, they probably will have some advice on how much preload those bearings can take. That might be useful. In general though, it's based on an installation that has zero flex, and the installer/builder has to add that into the calculation. In practice, if a bearing that size is preloaded to half a thousandth of an inch, maybe to one thousandth of an inch, it'll be in a very happy place. But I don't know at all how much flex is going to be in that headstock casting. Nor do you probably. But since I'm gessing you don't have or want the tools to measure that in order to throw it into any calculations..... Well, that's probably why the manufacturer stuck radial bearings in there in the first place......
So to set that up by "feel", what one might do is something like this- Put it together, put the nut that tightens the whole thing on, and while manually rotating the spindle, snug the thing up until you can just feel the bearings start to have some drag. You don't even need to quantify it, just barely perceptable is plenty. Then run the spindle first by hand, five or ten turns, or whatever... feeling for absolute smoothnesss. It should be. Then snug it down right tight, a little at a time, turn, tighten, turn, tighten, until the spindle offers significant resistance. Maybe half a turn if you spin the chuck and let go. It should still be smooth, even though you are (on purpose) making it way too tight. What you've done here is prove the bearings aren't crunchy, seated the bearings properly, just in case the installation was not as perfect as it could have been, and basically just verified everything about the installaton is just right, prior to adjusting.
Now, back off that spindle nut, and bring it down to just snug. Hand tight maybe, but all you can give it. (Yes, that's broad range, but that's OK, it's quite close enough, it's just a starting point.) Then, under power, run the lathe for a few minutes, feeling the headstock as near as possible to each bearing. If it starts getting hot, you're too tight. If it doesn't heat up, you havn't tightened it enough.
Generally, for light applications like this, the IDEAL place to be is right at or just below the point where you can JUST BARELY percieve a temperature change after 20 or 30 minutes of run time, If it warms up tangibly, back it up about a thousandth of an inch. If you havn't felt any temperature change, tighten it about a thousandth until you do just barely percieve a temperature change after that time. Personally, i would be tempted to run these SLIGHTLY on the loose side, which means I would find that "just barely perceptable heat" point after 20 minutes of run time (at a middleish spindle speed), and back the nut up one thousandth of an inch, and call it good.
How do you measure a thousandth of an inch of preload in such an application? With enough metrology equipment that it'll cost more than your lathe... So you don't. That spindle nut and spindle have some thread pitch. I have no idea what it is, but I'm gonna throw out 20 threads per inch. (You're gonna have to convert anyhow, why not.....) So at 20 threads per inch, the nut advances fifty thousandths per revolution. 25 thousandths per half a revolution. I think you're going to have one of those four slot spanner nuts... That makes it easy. 12 thousandths per slot. Call it 10, and envision divisions on there. Call it ten even, for easy estimation. That's close enough. That's about the coarseness of the adjustments you want to make in that case, about one tenth of one quarter of the nut. Draw marks on it if it helps.
This will NOT make high speed spindle bearings for CNC equipment, but if you get it right at that spot just between not making heat and barely warming up at all... you'll be good.
And let me assure you, this is how I personally would set up a bearing. Other people have other methods, and that's fine. So long as you get it in a happy place at the end, it'll all work out just fine. When you're not maximizing the load of a bearing (you will not be) the window for "good" becomes (relatively speaking) a pretty big margin of tolerance. Give it some time and effort (mostly time for those 20 minute run time windows...) and you will get this just fine, even on the first bearings you ever set up.
