For the 30mm UD using Ruger oem rings, you'll need 1 medium and one high. No low avail in 30 mm unless you go Warne who makes a low and add amatching low but would need a base mount like Weigand if you do.
I'm using a Burris base mount for their Fastfire II on my 480 SRH (see that there is a III waiting in the wings) open style but haven't an opinion yet. My other 454 SRH hosts an 30mm UD in the aforementioned warne/weigand setup