I recently did a neck reset on the FG150 but didn't take it back far enough. After stringing it up, the caving of the top of the soundhole pretty much canceled the neck reset adjustment. So I put a new brace on the back and set a post between that and the brace above the soundhole. What that did is it prevented the top from caving and that fixed the reset problem. I can now put a saddle in that has room for lowering in the future.
The stick measure at the bridge was about 1/8" above the bridge without string tension. But once I glued the neck in and put full string tension on it, the neck came forward due to the caving of the top. Now with the post that doesn't happen and the stick measure is about 1/16" above the bridge.
The Yamaha still sounds great, huge highs and midrange, like a taylor, without a booming bass. I know the post would never be a solution in a Martin, but in these old Yamahas something has to be done. These guitars are great guitars that I sometimes take to a jam instead of my Martin if I know there will be a bass player. These are LOUD midrange guitars.
That is odd. I've completed (69) vintage FG neck resets and have never had this happen. There's a brace under the fretboard that should prevent this, although 50 years of string tension will cause the top to sink in around the sound hole, unless the brace has come loose.
When setting the neck angle, I take measurements with the guitar strung up, clamped in a neck jig, tuned (2) steps down. Then I don't get any surprises. There is a certain amount of flex in the top and bracing, which is probably what happened to you. You can't measure the neck angle without strings and guess where it will end up with strings. My final result is always within 1/64" of where it was while clamped and unglued. But I've had a little more experience.
I forgot to ask. How was your experience removing the neck??
I've found there are varying degrees of difficulty. A handful that are easy or brutally hard, but most are fairly difficult to remove without damage to the neck or the guitar body. Every one is full of unknowns, since you don't know how much or where the glue is. I'd done (3) where there was no neck pocket!! The neck dovetail was up against the back of the neck pocket! There's normally at least a 1/16" clearance.