Don't just eyeball it. If your guitar is a well known brand you should be able to look up the string spacing online. Or you could even email the company and ask. I don't know if you'd get a response, but its worth a try. If that doesn't work, you can measure the width of the neck where the nut should go and use whatever is the closest known brand reference. For an acoustic its probably either Martin spacing or Gibson spacing. If all else fails, a Stewart MacDonald string spacing ruler might do what you need.

Interesting, all I use is a tiny drop of C/A glue, ( my Luthier does the same) and the main advantage is the time, it's almost instantaneous, I just hold it down for 5 minutes, same with the saddle, & double/check their firm& in place then string it up 😮😅😊

Nice and informative description of the possible errors that can creep in on a sanding job you described, that's why Iv'e found it best to use an Ozito ( or something similar, just a hobby brand) 2 stone grinder- Coarse and Fine, for finishing the nut and saddĺe, it can Still go Horribly Wrong, & Very quickly, but with care, Iv'e found it much easier, kind regards Don 😮😊😅