The AAF and RAF 100mph gunsight rings are 101mil and 105mil in diameter. It's called 100mph due to the relationship of bullet travel time to 1000ft(333yd) and how far a plane travels left to right at 100mph.
Keep in mind 300m = 328yd, while the German Revi main ring is 100mil. And there are still arguments at WW2 tech blogs about AAF, RAF, Luft, Finn and VVS Mil unit real size.
Effectively in 1\3 of a second a 100mph plane will travel across your 101mil main ring while you look through it, 50mil or 1 radius. 1\3 of a second is the travel time for a .50cal or hispano 20mm. When you lead a plane you estimate your lead from the center pipper or cross for the Revi.
.50 cal time to 1000ft +- = .3 sec.
30mm time to 1000ft +- = .6 sec.
A con traveling left to right at 300mph for the .50cal is then lead starting from the pipper by 3 radii or 1 1\2 101mil main rings.
A con traveling left to right at 300mph for the 30mm is then lead starting from the cross by 6 radii or 3 100mil main rings.
You can see this offline with the LCG enabled when you fly across the circle to let your con travel right to left in front of you when the icon switches to 400. SkyRock's recommendation to kick some more lead takes care of the +- factor.
Sadly using the fuselage length of fighters at distance to estimate hold off is dicey. The P51D at 1000ft is 33mil or 1\3 of the 101mil main ring. How many 1\3 of the main ring will it take to estimate for a .50 at 1000ft, then a 30mm at 1000ft? On the other hand the Revi16 reflector plate in the K4, Ta152 and Me262 is about 150mil in diameter. Using only a dot, that is one huge gunsight main ring using the reflector plate edge. And the slower the round, the more lead your main ring needs to account for.
So using only a dot, your lead at 300yds becomes two Revi16 reflector plates shooting at a con flying 90 degrees at 300mph. Or you can edit the mil file in my historic pack gunsights with 101 and 100 mil main rings. Change the top line from 256 to (384 or 400) and that will up size the main ring. The LCG will reflect this if you test the 30mm offline.