I haven't dealt with the low turrets on the Ultra Shorts yet, but on the older DT and MTC/LT turrets even if the elevation adjustment mechanism itself had say 31 mils of usable travel the turret knob limited you to 26 mils of usable travel since the knob would not rotate past 2 revolutions. I run a 44MOA base on a regular 3-20 on one of my .22's for messing around at longer ranges (for a .22, anyways) and it works fine-- but I'm limited to 26 mils of dialing because that's the two-turn rotation limit on the MTC/LT turret.
With the Ultra Shorts, I see there's a 17 mil double turn turret available, so if your 3-20US has the 17 mil DT turret that means the knob would allow 35 mils of elevation based on the pictures I'm finding. So a 40-50 MOA base should be just fine.
One thing you may want to check for before going this route is degradation of the image quality and possibly the eyebox when dialed near the extremes of travel, and if that degradation in image quality bugs you. The S&B 5-25's vignette and lose a little image quality when dialed near the elevation extremes, and my S&B 3-20 doesn't vignette but does lose a little image quality when near the elevation extremes. Eyebox may get a little bit more touchy, but it's a minimal effect if any.
On an ELR rig you're probably going to spend most of your time dialed up into the middle to middle-upper end of the elevation range on the scope and not spend enough time dialed down to the bottom of the elevation travel at your 100Y zero for the degradation in image quality to be an issue.
Another thing you may want to consider is something like an cant-adjustable mount from Era-Tac, they're adjustable from 0-70 MOA and you can dial it in to exactly what you need to max out the scope.
