​Busy timer not correct

A negative time in the countdown happens when LightBurn underestimates the time required to finish the job.
Go to Edit > Device Settings > Additional Settings , and click on ‘Read from controller’ (while your printer is connected and powered on), then click OK. This will try to read values from your controller and update the values used by the simulation/estimation, you can also manually edit the speed scales yourself, to try and further improve the timing.