MIMF

So I have come back to making guitars after a long stretch. Long story short I had an old dish labeled 15’ back but it clearly is a 25’ dish. I sanded my assembled sides in it
I noticed it when I was sanding the curve into a brace and it just seemed wrong. I dug out the correct one and see that I will have to reduce my sides by about 3/32 in height if I sand on the correct form. I’m building a Martin style dreadnought. If I proceed will the sound and/or volume be compromised much? Would it be crazy to add material at the waist that will be covered by binding? Dang

Nothing to worry about.

Resand using the right dish and then install your linings "proud" of the sides and you won't have to lose any depth. I would install the linings about 5/32nds proud and then touch them up (1/16th) on the dish again. Have you added in the additional height the back adds to the side depth?
Even if you just proceed with what you have, as Aaron mentioned, 3/32nds won't make any difference in the overall scheme of things.

Alas, the linings were already installed and sanded in the dish before I figured this out. Yes I have taken into account the top and back thicknesses and stopped sanding when the headblock and heel were at the calculated height. Nobody took a bite on adding material over the linings at the waist? I suppose it would be trouble if anyone ever had to pull the back off someday.

I wouldn't worry too much about either 1) the small loss of depth should you re-radius or 2) having a shallower dome in the back should you proceed without change. I would be a bit weary of adding material to the rim as that could turn into a mess for very little benefit. But if the linings were not already in, the advice to install them proud is good.

FWIW, I typically use the same radius for the back as I do for the top. I'm no expert and I don't make dreads but I have not found any issues with the shallower back radius. Perhaps others will post info on the cons and we will both learn something.