Hello Robb,
Attached is a formula for fill pressure and our reccomended fill chart. You must keep in mind that the temperature you fill a tube will effect the ultimate fill pressure. We reccomend filling at 50C / 122F because this is the typical running temperature of the tube. You must also keep in mind that tube length will also be important to factor in as well. As the tube gets shorter the fill pressure must also be increased to compensate for resistance in the arc stream.
Your standard size glass for channel letters is 13 and 15mm. On larger letters 4 ft and up you can use 18mm glass.
I am a glass bender, shop owner and manufacturers technician. if you have any questions you can PM me.
Have a great day