Don't sweat it, as long as the strings clear the first fret it will be OK. Once you've got that done you can tinker with it until it's as you like it. Making a simple guitar is a practical task for a practical person, so I'm sure this will be the least of of the things you have to deal with as a first-timer. Good luck, enjoy the process and don't worry about it too much - use your hands, eyes and commonsense. I've made nearly 1400 cigar box guitars, so I think I'm in a position to say, "don't worry about it"!
When setting the string height over the first fret there are a couple of things I would suggest to keep in mind, even for a beginner. Are you playing fretted notes only, or using a slide. Do the strings have a low tension favored by players that fret notes and cords, or a higher tension favoured by many slide players.
If playing fretted notes max .020 is as high as you need to go I recon, that's electric guitar setting, acoustic guitar max .024, but Id start a bit higher and cut the slots deeper to suit your feel. If playing slide, thinking about string height, slide weight, and string tension will help avoid rattling of the slide on the frets.
The slots should be just deep enough to have at least a third the string diameter above the slot/top of nut.
Having watched a mate of mine build two CBG guitars, I can better understand the challenges faced by newcomers. As mentioned enjoy and don't sweat the small stuff. I'm sure this is the first step to a few better instruments.
Hi, a safe way to get the slot depth, ball park first time, is to stack the required number of feeler gauges on the fingerboard , to match the fret height, then put on top of that stack the feeler gauges needed to get the required distance from the fret to the bottom of the string/slot. Slide this pack up to the nut and file down till the file just touches the top feeler gauge. Fine tune from there.
The nut action I mentioned earlier relate to the bass/ wound strings, on a six string guitar I would normally go lower for each string 6 down to 1, There is less movement in the higher vibrating strings so I set them closer to the frets. To give you an idea I set acoustic's at .020 to .012
don't overthink it.
if the slot is too deep the string buzzes on the first fret, if the slot is not deep enough your fingers will ache trying to press the string down to the first and second frets.
press the string down between the second and third fret,
if the string touches the first fret you went too far, but if the gap between the string and first fret is big enough for two credit cards, you ain't gone far enough.
inbetween is comfort, preference, and opinion.
Hi Larry, here's another tip to help from going too far with the slot or judging the nut height, cos going to far on a bone nut can work out expensive.
Get a pencil long enough to lay along the first 2-3 frets, then shave and sand it in half lengthways, so the lead is just about showing along it's length.
Then lay the pencil on the frets move it up to the nut and make a line on the nut, this will be the height of the top of the frets. keep your slots above the line and you should not go too deep.
I've been using the same pencil for about 25 years for this job.