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