The arc is the angle change of the rod butt during the stroke, and the bend it has to accommodate is the bend the rod actually takes. That bend is not a property of the material label. It is produced by the line being carried, the force being applied and the way the rod behaves under both, so it changes with the cast in front of you.

“Graphite” or “bamboo” may tell you what kind of bend to expect, but it is not a measurement of the bend happening now. Two casts with the same rod and different amounts of line can need different arcs; the same length of line cast with a softer or sharper application can do the same. Match the arc to the bend you have, not to the material’s reputation.

The loop is the gauge. If the arc is too wide for the bend, the loop opens; if it is too narrow, the loop can tail. Adjust the arc until the rod tip path is straight and the fly leg unrolls straight. That is a measurement you can make on the lawn, with the rod in your hand and no label to read.

Source

From the mechanics discussion in the MCI Study Guide (March 2023), used here for supporting mechanics rather than CI rules; exam material is in the CI Study Guide (2025); terms follow the FFI Fly Casting Definitions.