Wiring Up New Studio and Matching Levels

[SoundBoy]

Ok so im looking at how im going to do the connectivity in my new studio. I have a Allen and Heath GS3000 (24 ch) recording console and a focusrite liquid Saffire 56.

What I want to do is connect this up to record through the console and then be able to come back to the console and mix "out of the box" and record the mixdown again.

So to begin, signal into channel 1 on console - channel 1 tape out to saffire in - saffire out to channel 1 tape in. Repeat for chanel 2 etc.

I plan to run 8 mic intputs into the first 8 channels on the console from there to mic in on the saffire. then 8 line ins (from di) on chanels 9 -16 on console then to saffire line in. Then since the console has inline channels the first 8 channels will have the saffire feeding back into them. (this will be expanded later with more channels coming back into the console using i.e. octo pre mk2 light piped to the saffire)

I then plan to have the monitor outputs from the saffire going to the 2track input on the console.

So as it stands I can record up tp 8 mic or line inputs through the console into the saffire, i can monitor/mixdown 8 individual channels back from the interface into the console. Now the question is, how do i record that mixdown in stereo.

Do i have to sacrifice two of the line inputs on the saffire? e.g. console 2track out to saffire line 1 and 2 in? Or use a patchbay maybe?

And my last and probably even more important question. How do i match my levels on console and daw?

I would like the DAW level to sit at around -18dBFs when the VU on the console is at 0. The headroom of the console is +21dBu and the saffire max output is +16dBu

Not sure if im understanding it correctly, how do i go about testing and setting these levels? The console does have a built in sinewave oscillator. Could I use that at 1kHz to match the levels?

Thanks so much for your help

G