If you can measure diameter (not trivial if you have a 3 or 5 flute mill and don't have the corresponding mike, but you can do it by laying it on a surface plate and squashing it (figuratively) between two square pieces of metal and measuring the inside dimension) and you can measure the pitch (follow one flute all the way around and measure the axial distance along centerline from one point on the helix to another) you can calculate it pretty easily.
If you label cells as Pitch and Diam in Excel, the formula I came up with is
=360*atan(Pitch/(pi()*Diam))/(2*pi())
If you have a 1 inch diameter, and a pitch of about 2.4, you get a helix angle of 37.4º. If I've left out a factor of 2 or something, I'm sure I will be corrected.
Having said all this, Mr. Kinsman's method is clever, practical, and probably gives you a better idea of what "helix angle" actually means.
Good luck,
Jim