How to cut a plate onto a tube at an angle?

[Drpia]

TL;DR Summary

Formula for a plate to sit on end on a tube at an angle

I'm trying to draw a file for a plate that will sit on a piece of 6inch tubing at an angle. I can't quite get the radius right. I've tried
R=(.5mM)÷square root of (m squared + M sqaured) m=minor diameter M=major diameter
And I've tried R=(H÷2)+(W squared ÷ 8H)
H= height of arc W=width of arc
The second one gets close but its still not quite right. Is there another formula for this?

 

[DaveC426913]

I'm sure there are other ways. I'm not quite sure where you are running into trouble.

Do you know the angle of the plate to the tubing?
What result is it that you are looking for? The properties of the ellipse, drawn on your plate?

Obviously the minor axis will be six inches. You should be able to work out the major axis AB pretty easily using soh-cah-toa:

For example, if angle α is 30 degrees, that should make for a major axis of 6.928":