A friend of mine once got an A+ grade at an oral examination being asked how a certain technical construction should be calculated.
The professor expected the answer to be referring to a complicated mathematical nomogram developed and presented during his lessons, but my friends answer was: 'The construction should be calculated to bear the load it was intended to take'. The professors reaction: 'Excellent!'.
On the jimer document, there is a formula had been given for the disc thickness on butterfly valves. Actually, it seems mental but there is no reference was given for that formula. We need to have the source. If anyone have this kind of information, could you provide us, please?
BTW, It is not for an aim, just for general information.
In the jimer document, it states the formula is based on a simply supported flat disc with uniform load. It is clearly a rework of a Roark formula to solve for thickness given an allowable stress. To get the formula, get the textbook "Roark's formulas for stress and strain".
To supplement bcd's answer about Roark formula to calculate thickness for a given allowable stress, I would also calculate deflection at the center of the disc. Too much deflection will bow the disc and obviously loose tightness, eventho. you may be within the allowable stress.