There is a scale on the side of the Chart View to compare it with.
You really don't need to use that for distance.
Use the Casting Rings setting at the radius setting for the casting distance you want. Stop when you get to the ring.
At the 10' max zoom on a Helix 12 the course projection line is a little less than 30' in length.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1