very good point - you will need slightly MORE lead for 20mm cannon and even more for 30mm and slow ROF guns, but this will give you a ball park to start with and in my case my perceived lead was at least 1/2 a ring too small in most cases and 1 full ring in high angle off shots.
the big eye opener was how a little back pressure on the stick has a drastic effect when turning and trying to get lead- I now fly with nose trimmed well down
best to keep you Angle off less than 30 degrees where possible to have a chance of hitting.