Yes, between the 20 and 80 would be low. You want to be between the 2 marks on either side of the 80. Add fluid slowly. It won't take much.

I just did this to my 2000 C280. It would go into limp mode after a few stops typically after just getting off the freeway. When stopped, moving the shifter resulted in really harsh shifts. Restarting the car would reset it. This worked a few times until it was stuck in 2nd for good.

If you don't know the service history of the car, go ahead and do the connector along with a filter and gasket and some new fluid. You might get lucky and this is all it is. If not, then you've wasted the fluid and next is a new conductor plate.

Use MBZ connector only, only about $12 on ebay. You can get the plate there too for about $130. My old plate showed no obvious fault.