That’s fairly easy, though it depends on whether your controller uses “step size” or “steps per mm”. They’re used for the same thing, but the calculation is a little different.
80 steps per mm is 1/80mm per step.
If you tell the machine to make a cut that is 100mm across, but it comes out to 150mm, your ratio is 150/100, or 1.5.
You would divide your steps/mm by that number, or multiply your step length by that number, to get the correct answer.
So, if your machine used 80 steps/mm, you would divide by 1.5 to get 53.3333 steps per mm. If your machine used a step length of 0.0125mm, you would multiply that by 1.5 to get 0.01875mm per step.