Stouffer 21 step grayscales are far from worthless. I'd say they're the most valuable thing in a home darkroom.

Coupled with a Macbeth densitometer you should be good to go. Sure there are zero and calibration adjustments. I'd stay off them unless you have the instruction manual and a calibrated patch.

But just because the first step of 21 steps measures .20 instead of .15 isn't going to end its value. Check what difference reads between each step. That's where you should "average" .15 between each step. If you are even close to that +/- 0.02 is real close.

For example (zero'ed on air) my 21 step Stouffer wedge reads: .04, .18, .34, .50, .67, .81...