I contacted Karl Miller, who was at that time involved in the daily operation of Miller Clay, and he suggested Miller 570, a clay with higher firing temperature but still based on Miller 550. It was a step in the right direction, just not a large enough step.
So I asked Karl to help me design a porcelain formula which could be fired to cone 12 without sagging, but still based loosely on 550, and this is my final iteration, some 20 years later.
Custer Spar 15
200m Silica 28
Bentolite L 1.5
VeeGum T 1.5
Plus add 44 grams of Epsom Salts dissolved in hot water for each 100 pounds of dry mix.
I do not know the amount of water that is added to the formula, but if I were making it myself, I would determine the correct amount of water. And it would be pure water.
Notice how this clay differs from the so called "Equal Parts Body", which is 25% each of Kaolin, Ball Clay, Silica, and Feldspar. It has way more clay, more silica, and less feldspar.
My celadon glaze, the 4, 3, 2, 1 glaze from Leach, does not craze or shiver on this body.