As for the maximum DC, keep in mind that if you are using HGE, you have access to Foebane spell which adds "+2 on the next CS attempt". And due to your high wisdom (getting close to that of top end casters) and equipment, you will likely have plenty of slots for it.
I do not consider foebane worth memorizing, simply because a failed CS will drop (most) enemy con by 6, resulting in 3 less fort. The extra +2 DC is not worth wasting a valuable spell slot and an entire round casting as I cannot think of any time where 1 CS attempt at +2 is prefferable over 2 CS attempts at +0 (with the 2nd one possibly at effectively +3). And for the argument that you can prep ahead of time before a spawn.. it's still a valuable spell slot, and that would mean you're hitting an uncursed mob at +2, which where it counts means you might as well be at -6 or more, and won't kill it till the 2nd round anyways where it'll be cursed again (and you'll have lost that -2, but gained the +3 from con drop). As a ranger, you have nothing close to caster spell slots, and I find many of my rangers still wearing the UR belt because it gives +3 spells per circle (vs +2 from any other item, including rona bracer and BUR amulet). Admittedly a zen ranger will be more likely to be able to spare a few 4th slots for foe-bane vs a melee, but I'm far from convinced that it is worth it.
Now if Foebane were more like 1 minute+ duration for EVERY called shot during that, it would be a much different matter.