Two standard conventions:
1. Write the
field angle in the symbol on the
lens tube, (
Vectorworks'
stock symbols follow this, but the lettering is small and gray, so many people edit the symbol to make the
lens designation more prominent).
2. The USIIT Graphic Standards is as follows:
4.5x6.5, 50° - a wide "V" in the
lens tube
6x9, 36°, 40° - blank symbol
6x12, 26°, 30° - single slash through
lens tube
6x16, 19°, 20° - double slash through
lens tube
6x22, 14°, 10°, 5° -
line parallel to
lens, or two short lines perpendicular, or use larger/longer symbol.
See
Prefect's Pages.. Publications .. IQP .. Instrument Plots, or
USITT Standards website, for more information.
edit: I misunderstood the question, as the OP mentioned drafting.
What I did when I had a flock of
Colortran Mini Ellipses, which had movable
lens, was just to write the degree on a piece of white
gaffers tape on the
yoke near the
C-clamp. Saved a lot of frustration (with the beamspread, anyway).