Deutero,
Since you're a detailer, you probably can do this. I did it with autocad.
1. Draw back of web (vert line)
2. Draw outside face of flange (hor line)
3. Offset flange face by distance k = 1.125" (this gives hor centerline of fillet.)
4. offset back of web by 0.282" (this give inside face of web.)
5. offset that by R = 0.50". (this gives vert centerline of fillet.)
6. draw line through intersection of both cl's at slope of 1:6. (JStephen is correct about slope.)
7. Offset this line by R = 0.50". (this give inside face of flange.
8. Put in a fillet of R = 0.50" to the inside face of web and inside face of flange.
9. Measure or dimension the distane from the intersection of fillet and inside flange face to outside face of flange, I measure 0.6318". (Slightly different from SlideRuleEra's value from 1923 Carnegie.)
10. Also, offset back of flange by g = 1.75"
11. measure or dimension distance between outside face of flange and inside face of flange at this location, it is 0.4568". That is closer to 7/16" than it is to 1/2", but maybe they round up for the grip.
12. offset back of web by 3" to find tip of flange.
13. Measure or dimension distance between both flange faces at this location. I measure 0.2484". (Slightly different from SlideRuleEra's value from 1923 Carnegie.)
13. IFRs is correct about the radius on the tip of flange, but I do not know that value.
I think that gives you a good enough picture of a C12X20.7
regards,
chichuck