There may be some confusion in units here, but I may have missed it if this was already known or mentioned. The gcode spec dictates angular or rotational axes will have motion specified in degrees, not mm. Thus, G1 Annn.nn is a motion specified where nnn.nn is degrees, and can be positive or negative. The grbl configuration setting $103=26.660 in your case is a specification in steps/degree. Yeah, its different from the other axis values of steps/mm. The conversion from mm to deg in the LB drawing is handled by LB, that’s why LB wants to know the diameter or circumference of the cylinder (work area) held by the chuck, not the diameter of chuck, the diameter of the cylinder held by the chuck.
If you like running with grbl Soft Limits ON, (I do but mainly for XYZ reasons), then after you Home your machine, move the A axis to midway between 0 and the max degrees value, $133=6000 in your case. I like to use multiples of 360 for my max deg value, 2160 is nice one giving 6 full rotations.
Net: $103 value in steps/deg, $113 in deg/min, $123 in deg/sec/sec, and $133 in deg.
One more thing, several grbl implementations have a bug in that the feedrate is not correct when motion involving a combination of linear and angular axes is commanded in G94 motion mode (distance/time), such as G94 G1 X10A10 F1000. G93 motion mode (inverse time) works fine. The bug has been fixed in grblHAL, see more info here: Where in the code is G94 G1 XA feedrate situation assigned to X while A is coordinated to move with it · Discussion #241 · grblHAL/core
and here: Seeking to verify rotary setup is correct - #13 by LightBurn