1/TPI = vertical distance traveled (inches) in one turn. Always round that number up, so you seat too shallow. It’s easier to add more turns than to pull the bullet and start over.
Standard dies are 7/8-14 = 1/14″ per turn 0.01″ per 1/8 turn is a healthy over-estimate.
Lee seater stems: use +0.015″ per 1/4 turn to estimate. Actual number is 18TPI and closer to 0.1388…89″.
Redding 3BR Pistol Insert
Whole numbers probably represent 0.01″ (0.254mm) of micrometer travel. Micrometer spindle hole is approx. 0.314(5)” when measured with calipers. A .312 (Likely 5/16″) insert enters hole. 8mm = 0.314961″ is a working metric approximation for the hole’s diameter.
The empirically-derived models I have for predicting a needed setting with a new powder are:
grains = insert_number * 0.0115 * VMD ^ -1.039 + 0.29.
insert_number = (grains – 0.29) / (0.0115 * VMD ^ -1.039)
grains = insert_number * (0.2627 – 1.249*VMD) + 0.29
insert_number = (grains – 0.29)/(0.2627 – 1.249*VMD)
I still haven’t tried them with a new powder.