When we had a 2x6 we used 3/4" pipe. The head tank was about 3' above and 2 feet behind the evaporator. We had a T with plastic sight line pipe to show the head tank depth.
With our 3x19 we use a 1 1/2" pipe because the inlet is 1 1/2". The head tank is about 4' above the evaporator hood with preheater. We have a sight line tube installed near the head tank. The evaporator has a up tube to help address vacuum issues at the hood. The feed tube is about 24' long to wrap around the outer edge of the room to get to the inlet.
I think the goal is enough to feed in for emergencies or draw offs. A consideration for ease of operation is a site level and if the run is short enough it can help address vacuum issues.
Mike
Tapping since 1985 (four generations back to early to mid 1900s). 200-250 taps on buckets and then tubing in the mid 90s. 2013- 275 taps w/sap puller 25 gal. 2014-295 taps w/sap puller 55 ga. (re-tapped to vacuum theory) 2015-330 taps full vac. 65 gal, 2016-400 taps 105 gal, 2017-400 taps 95 gal. 2018-additional 800' mainline and maybe 400 new taps for a total near 800 taps. 2x6 Leader WSE (last year on it) supported by a 250 gph RO.