It's the length of the vibrating string, so the fingerboard edge of the nut to the forward edge of the top/high E saddle. The T-O-M bridge is slanted, so only the high E string will be at the scale length (and after being intonated it may sit very slightly back from the nominal scale length by a very small amount, between 0-1mm). So measure from the high E slot to the high E saddle, as that's where the string will run from/to. The saddle can obviously be moved, but you'd want the high E set somewhere near the front of the bridge, as the other saddles will normally be set further back than this when intonated.
The slanted bridge helps with the small saddle adjustment range of the original ABR-1 style Tune-O-Matic bridge (which the AG-1 kit comes with a copy of). The later Nashville style T-O-M bridge has a couple of extra mm of saddle travel. If, when it's built, you do struggle with intonation, then swapping to a Nashville bridge is an option (though I've never had an issue with intonating an ABR-1 style bridge).
About to set the neck, having read a lot of issues with action height I am need a bit of a hand. SO what I have done, scale lenght set to 628 mm (24 3/4") at high E. Neck is currently straight no curve. Is visually straight with high and low E fitted and space for pickup is OK fits with no gap front or rear.
With the heel clamped flat in the pocket, I see action height of + 6mm at the body end, at the nut end approx 3 mm, still have original nut in place will replace with bone nut. Saddles are set as low as possible.
I am thinking the neck angle is not right and needs to be lowered at the head stock, my plan is either to sand the heel (very carefully) to give me this angle, or place a wedge shaped shim at the back of the pocket to give me the angle needed. Still undecided best attack plan here.
So put a shim in (still not glued), clamped up and now have this result, shim is 3mm to 0 ish set high end to the body, seems to be looking ok, action is a bit low, but leaves me lots to adjust with saddles as low as possible, neck is flat and nut yet to be worked, along with frets dressing etc.
Some of the final finish on the body and neck. I used - Shellac - mixed at roughly 125gram of shellac to 1/2 L of Metho, about 20 coats over 10 days, light 1200 wet and dry sand after each 5 coats. Then used water based poly - wiped on, 50/50 mix of brush on thinned with water, this took a bit of trial and error to get finish I wanted. 5 or 6 coats of poly wet sand with 0000 steel wool between coats. Final coat of poly let dry for 4 days then used a fine grade cut and polish (maguiers I think) and lots of elbow grease, came up ok. HAs a few marks and minor bumps and swirl marks, I could probably work a bit more on it if I wanted to to get it better, but was keen to build it.
The strings look a long way from the body at the bridge end, though I can’t see the bridge in the photos. How high was the bridge set?
Also, clamping only the tenon and not the end of the fretboard as well, you can force the headstock end up a bit, and even a small change in neck angle in that direction can raise the action significantly (for a given bridge height).
You’ve now got a nice shiny body, so there’s nothing stopping you inserting the bridge bushings so you can fit the bridge properly (not forgetting to first install the bridge grounding wire) and get a better idea of the action with the bridge set at its lowest and seeing where a straight edge set flat along the fretboard hits the bridge area.
That all sounds pretty much spot-on. Should be perfect once you get the nut sorted with a little bit of height needed on the bridge. It's good to be able to lower the bridge slightly from your ideal height, rather than have it flat on the body, but you don't want it sitting too high.
The higher the bridge, the more it can rock back and forth due to the play between the height adjustment post screws and the inserts. They are always looser than normal threads to allow for a bit of misalignment in the hole positions and angles. The more thread you have making contact, the more stable the bridge will be.
The strings themselves move the bridge back and forth as they vibrate, as the string tension changes at double the string frequency, and the tension changes try and move the bridge each time. I'm not 100% sure of the effect on sustain, but I just feel happier if the bridge feels steady and can't be moved.
If very loose, I tend to wrap plumber's PTFE tape around the threads to take out the slack. The tips of the threads poke through the PTFE and make contact with the inserts so the bridge remains grounded.
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