It seems pretty much standard for conventional guitars to have fret position dots at fret numbers 3,5,7,9,12,15,17,19,21. Five string bluegrass banjos are marked almost the same, but a banjo differs in that its fingerboard usually marks the 10th and 22nd fret instead of the 9th and 21st. The advantage of marking a fingerboard like a banjo instead of a guitar is that the pentatonic blues scale will fall right in line with all the dots on the root note string. Every dot will represent a pentatonic note, not the case on a conventional guitar.
With a cigar box tuned to GDG, you have two outer root note strings and a middle string tuned to a 5th. This means that the whole neck will be marked for the pentatonic blues scale if you use banjo markings instead of conventional guitar markings. Remember that the open notes are pentatonic, and you will know right where all the good soloing notes are at a glance. The only exception will be on the middle string at the 7th and 19th frets. But, not to worry, these are still strong soloing notes since they will be the 5 note of the 5 chord(the power chord side of the triad). Plus, it's easy enough to force these notes pentatonic by giving them a good half step bend.
Anyway, that is why I marked my first build like a banjo. What do you think?