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I am starting on the second version of my mandola/octave mandolin. The first try was a bit mellow and I want to get more bite this time. I hope to have a youtube by September.
This version will use maple back and sides with an oval sound hole on top. The neck wood is not selected yet. Depth of the box is one inch which gets tricky aiming a hanger-bolt just right. Most guitars use an angle of decline of one and a half degrees to match a 28 foot radius on the top and I hope to a shorter radius (15 ft or 8 ft). Flossing a one-inch heel won't get you much.

I want to start out with what is normal and adjust that to what will work. Any comments would be appreciated.

Bob

On my flattop mandolins and mandolas I use a radius of 15' and a neck angle of 1.5deg. There is a bit of adjustment involved when I fit the neck, but that is what I start out with. My neck joints are dovetail for all flattop and arch top mandolins and mandolas. Quite apart from the difficulty of access I can't see any advantage in using a bolt on neck joint for mandolins, and I have done over 150 dovetails so it is no big deal. On the other hand, bolt on does have advantages in guitars and my guitars do have a bolt on neck.

Peter

It's going to depend somewhat on how much your fingerboard will be elevated (if at all). I'm building one right now that is deeper bodied, but otherwise similar to your concept. It's a flat top, with an induced arch and was built in a 15' radius dish. It also has a bolt on neck. My fingerboard will be SLIGHTLY elevated--I'll have a thin ramp glued between the fingerboard and the top. I just measured the angle of this incomplete neck, with the fingerboard taped on. No frets on it yet, but it looks like it'll have a good string line to the bridge saddle, with room for an adjustable bridge. My neck angle is a hair more than 3.5 degrees at this time.

I helped a friend build one a few years ago to these general specs and it came out very nicely. On that one and the current build, I contoured the sides in the radius dish before putting in the linings. Thus, the sides are tapered off in all directions and are shorter at either end, with the highest point about even with the sound hole. Not sure that makes sense, so I'll add a picture that will show the taper and also the wedge under the fingerboard.

Thanks for the help.
I am using a butt type connection and hanger-bolt fastener (I think). Using a compression-type truss-rod in a short heel has me wondering whether I can connect both the truss-rod and the connection-bolt to the same dowel-nut in the heel. It might take some planning to get the holes in the right place but it seems to make sense mechanically.

I will have the bare neck flush with the soundboard and I should use falcate bracing in my 8 ft dish to match the first version (my experience has been that the top never retains the full curvature of the dish).
I have cut heel at 1.5 degrees and the bridge will be built to match.

Between a wood allergy and a rush project, this may take longer than expected.

Thanks for the help.

Bob

I am usually in the 2degree range , but im flat tops as well

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This will depend on a few factors.
How tall is the arch on the top?
How high is the fingerboard off of the top, i.e. how thick is your extension block?
How tall is your bridge?