Ok, I picked the third scale row up from the ventrals, least chance of hitting anything unwanted. Major veins and arteries run down the middle of the ventrals, and kidneys and such are high up by the backbone.
Also, if you put pressure on the opposite side, it will force the stuck egg to the suface and you can see if anything else is in the way. If you see anything other then white, do not puncture.
Measure the distance from the surface to the middle of the egg and mark that on the needle.
Also remember, you do not have to suck the entire contents of the egg out. Take out was is easy to take out. just reducing the size is the benefit.
Remember, this procedure works best on healthy females that became eggbound from loss of water. not from adhesions caused by bacterial or other infections.
Also understand, if you do this, you lose the benefit of knowing the cause. A vet can test for the cause.
In this case, the vet took a quick fix approach and should be spanked. oxytocin or however you spell it, is to induce contractions. Lack of contractions is not normally the cause of eggs not being passed. Also, it was explained to me by a reptile vet, that the application of a steriod that relaxes the cloaca is needed in conjunction with oxytocin. Or the snake will be trying to push a big ball threw a little hole. Not the best idea. good luck FR