Voodoo Lab

So I run both knobs at 100% right?

Im using a mesa dual rect. with a gcx and a g major 2 going through loop 4
There are 2 knobs. send and return. If i turn send to 100%, what do i turn the return knob to? 100%? 0%? 20%?

Ive heard I can run my parallel loop like a series loop if i adjust
my effects loop knobs. What do I set the knobs at to run in series?

I have a gcx and major 2 Can I connect them by midi or just run through loop of GCX?

If anybody could help me that would be great.
I have mesa dual w/ 3 pedals, GCX, Ground Control Pro
I want to run the gcx through my effects loop, not up front.
Is this possible? How?