Driveshaft length using 1350 transmission and pinion yoke?

Ask the driveline company how they want the measurement to be done.

I'm the king of measure 20 times, cut once and it's still wrong so I was worried about screwing mine up too.

Mine was a TKX so the length isn't applicable but I just pushed the trans yoke in until it was touching the rubber boot but not squishing it. Then I measured center of the yoke u joint center to the u joint center of the rear yoke. When I got to the driveline shop the guy did a little of his own math from my measurement and it fit just fine when they completed it.