Definitely calibrate everything so your data means something.

If you want to see your mash efficiency, simply measure your grains carefully and your mash liquor, then take a gravity reading before adding any sparge water. That will help narrow down any pH issues.

If you want to see your lauter efficiency, measure your runoff volume carefully and take a gravity reading. That will isolate mashtun deadspace and channeling issues (unless you batch sparge, which doesn't allow channeling.)

Also keep in mind that the bigger the mash in relation to preboil, the lower your efficiency must be.

If I have a 10# mash and collect 7g preboil for a 5g batch, that gives me 40% more volume for rinsing. You have 8g for 6g batch, 30% more, and you're using almost twice the grains. In theory if your "rinsing power," i.e., the volume preboil:postboil factor for a given weight of mash, stays the same your efficiencies should be pretty similar. That would mean if I can get 1.050 from 10# with a 7g preboil, I should be able to get about 1.100 from 20# pulling 14g preboil. In practice, there is a curve — not to mention the point of diminishing returns — but it should illustrate why I recommend "working out your mash efficiency" on normal-strength beers and then taking what you get on the big ones, keeping enough notes to know what to expect.