There are so many factors to consider when it comes to mash efficiency. At the end of the day it doesn't really matter if you are hitting 70% or 85%, it's being consistent that counts. That way you can brew true your recipe, no surprises.
Here's my 2 cents worth when it comes to the Grainfather and mash efficiency:
1. Getting the water chemistry right helps. The times I have achieved highest mash efficiency is when I have hit mash pH of 5.2 bang on. Calculate your salts and acid additions on one of the water spreadsheets, measure your mash pH after 5 minutes recirculating, and adjust pH accordingly. I tend to hold a little lactic acid back from the calculations, because it seems easier to lower rather than raise pH. FWIW I did one brew at mash pH 4.8 and it didn't seem to matter. Remember to acidify sparge water to pH 5.5.
2. Grain crush size - the yin/yang of mash efficiency and sparging time. Can't use BIAB grain crush sizes for the GF. Stuck or very slow sparge will result. Too coarse a crush will result in poor efficiency. Crush size is also mill dependent. The only way to make an accurate recommendation would be to quote grain crush size percentages using the calibrated sieve thingies. FWIW I've had good efficiency (87%) using a crush size of 1mm with a Mashmaster Minimill (new fluted rollers). Sparge time was about 15 minutes.
3. Sparge technique. I've only ever started sparging within a minute or less of hoisting the mash tun. Haven't had a stuck sparge doing this, but I also haven't pushed the envelope with crush size.
I'd say find a range of techniques that give you an efficiency you are happy with and then refine in very small increments. For me, I'm now happy with my water chemistry and sparging, but will try dropping crush size eg. to 0.9mm, to see if this improves mash efficiency any more without creating too slow a sparge. Once I've found the right balance I plan to stick with it to achieve consistent brewing.