Hi David and welcome to the OLF..
Here's some tips from me and good on ya for resetting a proper dovetail. Old Harmony guitars are what people should be learning neck resets on because the dovetail is very much as Martin and others who may need resets too and are economically worth doing.
I like to line the straight edge up with the top edge of the bridge plus the fret height that the straight edge sits on. This is a very slightly amount overset and I do this intentionally since things will settle in and someone may use 13's on it too. So if your frets are .040" high the straight edge comes in perhaps .030" - .040" above the bridge top.
So be sure the neck is as straight as possible before using the straight edge and you said, thanks too, that you already did and do this.
I'll add make sure the neck is correct left to right too. I do that by making a mark on top of the bridge between the D and G and that's the bridge center. Then I use masking tape in four equal distant locations on the fret board, four is better than three and mark in the tape in the exact center of the fret board. Then my straight edge is registered on all four neck marks and the bridge center mark. Now your neck is perfectly straight left to right.
Last tip is we like to fit the dovetail as it's intended to do all the work mechanically and not have any dependency on glue for strength. The glue is there only to keep the dove tail in place and make you and I pull out our hair come next reset time

The lowest part of the dovetail is the business end the keeps the neck in place and not folding into the guitar under string tension. So very important to get a good, tight fit with the dovetail alone with no glue. We like it when a bit of a tonk is required with a fretting hammer to finally seat the neck in the dovetail. We do use glue but only to hold the dovetail fully seated. Avoid excess glue, always.
If the neck can be manipulated away from the guitar side cheeks manually especially at the lowest part of the dovetail snugger and better fit and shimming is required before final set. Fitting is an art of sneaking up on the perfect fit over time and with material removal from the shims.
And one more tip since I can't quit

. Mohogany shim strips we CA to the dovetail and then carve them back as we fit with CHALK

not caulk, sheesh what. geezer I am....
Good luck and again welcome and let me know if I can help you, I'm happy too.
PS: My first ever neck reset was a Harmony Sovereign and it all went great and the guitar was a nice player when done. Fond memories.