We recommend that you use ‘Current Position’ with rotary mode.
There is an explainer here: Steps/MM - LightBurn Documentation
If 80 steps/mm works perfectly when the machine is not in rotary mode, this $101 value needs to stay set at 80 steps/mm. LightBurn will scale the output when in Rotary mode when you use "Start’ or save the GCode.
For example, in the Rotary Setup, you told LightBurn that to turn a roller with a circumference of 31.41mm a full turn, the axis needs to be commanded to move 39mm.
So now, when rotary is enabled LightBurn will scale output to the Y-axis by 124%
As far as I know for GRBL, manual movement using the Move window or ‘Position Laser’ jogging controls are not subject to rotary scaling - so in your case if a 100mm jog move was sent unscaled to the rotary axis the actual travelled distance at the rotary will only be 80mm.
If you wanted everything to be scaled so you could use those jogging controls, then theoretically I guess you could disable rotary and ignore Rotary Setup, and instead manually adjust the $101 Y-axis travel resolution to reflect the 124% scale change yourself, (e.g. type into the console ‘$101= 99.3’) and remember to change it back to 80 steps/mm when you are done.