Driveshaft length tolerances +/-?

Make the measurement then subtract 1" for slip yoke stick out. That is the industry standard and has been for probably ever since cars have had slip yoke drive shafts. That's how I've always done it and they come out perfect every time.

So my Dart has SS springs (two righties), and it's sky-high in the back. I want to de-arch them about 1.5-2".

You reckon I'll need to recut the shaft?