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I'm doing a neck reset and need to reduce the top belly on a 1970 Supreme, (Harptone) 12 string dreadnought.
My question has to do with the thompson belly reducer cauls. I would buy this set but the sizes in the set won't work for
my guitar so I need to fabricate my own.

Now for my question, if you own the above would you measure the width and determine the amount removed toward the
edge of the top caul. As an example my caul needs to be 2" wide and it seems the radius should drop the edge down by
about 2mm or 1/16". This "looks" like the picture of the one sold, but I would like more accuracy.

thanks

You would think that after reducing the belly with the caul method that the surface will be no stronger than originally. What will prevent the belly from returning?

from what I understand you are steam bending the wood back. once the top takes the desired shape it should be good to go.
I don't believe steam bending significantly weakens the wood, this is as I've experienced steam bending. whether or not it holds long term
it a tricky question you are only altering the plate and a small area of sound board. I may after diminishing the bulge replace the plate, I already plan on
making a new bridge. its a step by step see how it goes process for me.

I still would like some input on the amount of curve, I plan on 1/16th drop on each edge for a2" width. Anyone?

You might want to consider this.
i just put one in my 40 year old Martin knock-off after replacing a cracked bridge.

It's called a JLD Bridge Doctor. I don't know what the general consensus is on this site regarding use of these. I know Breedlove guitars installs these at the factory on every new acoustic.

http://www.youtube.com/embed/l1wVTnkD_Is

I put a Bridge Doctor on an old imported 12 string, and it did the trick. I can't say what it did to the sound because it was not playable when I got it. Some people say it improves sound - I can't say, but it did fix the belly problem. It sounded okay, but it was not a great guitar to start with.
GS

Some one correct me if I am wrong but I think Breedlove designs this into the the guitar as a method to have a very thin and responsive top. I've played one and it sounded great. The reviews go across the board on the Bridge Doctors.

My experience with the JLD has been less than positive when used to actually reduce the belly on a guitar. They do indeed sound great on the Breedlove but on the Breedlove they aren't installed to reduce the belly, only to add some support (I guess) and to improve the tone. They always said it transferred energy from the bridge to the tailblock so improving overall resonance. On the breedlove they aren't tightened like you have to in order to pull a bridge down.

Long story, short. My experience was that sucked the tone out of the guitar, because it compressed the top too much when I tightened it to a degree that made a difference in the belly. I also cracked one once because I overtightened it.

I realize that's no your original question. I don't know how to steam a top like that, but in my opinion, the JLD is a poor option.