This has been around the US for years and has proven to be quite accurate.
"Velocity increases at one-fourth the rate of powder capacity" This is for a given powder and caliber/bullet and based on the same barrel length.
If your rifle does 3000 fps with a given bullet, and you increase the powder capacity 10% (quite an increase....) the velocity will increase 2.5% ,or, 75 feet per second in this case.
I heard this was wrung out and (again) proved in Handloader Magazine here in the US about a decade ago. Anyhow, I thought this might be of some interest to those planning guns.~Muir