@Braufather I was gonna ask what was the mash temp. As foxy says, I don't think 75% attenuation is too shabby, I dunno 1.060 to 1.020 sounds plausible for SO4 which is not a mega-attenuator anyway.
The other things I was gonna ask is:
1. What does your hydrometer read at 20degC in water? Reason for asking is mine reads 1.003 and even after many brews I still have to smack myself round the back of the head every time it says 1.020 and remind myself that's really 1.017.
------------- last time i checked it was bang on and the other half of the split brew hit 1011 with liberty bell
2. How are you measuring your 63degC mash temp and how long are you mashing for? I don't have a recirculating system (Klarstein Maischfest) so I stir the mash now and then and raise/lower the grain basket to even things out. But generally speaking the mash temperature is not uniform, plus I know the temp probe on my system is out by 1-2degC. Oh and of course the mash temp varies a bit over time as the thermostat kicks in/shuts off the heat - less of an issue if you only mash 30-60mins, but more so with an overnight mash. I sometimes mash high (70degC) to try to limit the FG, and I've a feeling you can get a similar effect if you mash low - but having the temperature wandering around doesn't help much with predicting your FG!
------------- 70 min mash, 10 min mash out at 78c. I have a braumaster and just by dropping from 65 to 63 seems to give me an extra .008 with a brew. i was doing it for that but to cut back on sweetness with maris otter and it works. i did a 4% summer ale at 63c, with maris otter with some wheat and pilsner, some sugar and it came it great ( was fearing it would be a bit thin but it had great mouthfeel) so have adopted 63 for maris otter.
Interesting you mention 70c, i am going to try 70c mash next to do a 3.9% pale ale, with a dry finishing yeast to compensate , but may use regular pale malt for that,