"I am trying to avoid the introduction of non-compressible gasses into the system."
An admirable Goal...

That starting the R12 through the line with it attached but on lightly....and then tightening it was the old style way of doing it...
I think they have new hoses ... which I don't have yet... which are designed to either avoid that or minimize the amout of R12 vented to accomplish that goal...
But I am surprised that that was not covered in the 609 test.. ???????

I agree that you are trying to do this with less tools than is required to do it properly or safely...
You can run your system many many ounces lower than the max charge with few bad results compared to even ONE ounce over charged.. that really works against the whole AC gas/liquid change in pressure/soak up heat theory...