George, John gave one approach to tuning your ADC on a star with this (don't know what link it came from)
<"...After you become familiar with it's operation there are a couple of ways to adjust the degree of correction for a monochrome camera. The one I tend to use is visually adjusting the ADC with an eyepiece set to be parfocal with the camera--- the amount of corrective dispersion an ADC produces increases with projection distance from the ADC so that's why the eyepiece needs to be at least closely parfocal. Another way is use of a W47 or similar filter that has 2 widely separated bandwidths, each at opposite ends of the imaging spectrum. The W47 has an intended deep blue bandpass but also has a near-IR leak, so when used with a camera with atmospheric dispersion present it will show a star split into two on your monitor---">
Most definitely his suggestion to me to place more distance behind the ADC for the dispersion effects to be corrected effectively works - we were imaging at about 40° elevation both times & the difference was very palpable!
As Grant said, you work with what you have (unless you go & spend more money on something extra! ) but I'm concerned re your: <"My assumption is that rotating the slip ring that the white screw screws into is what is changed to make the white screw level with the horizon.">
It's not rotating the slip ring that orientates the ADC for use, it is rotating the ADC body itself, & that ADC body's slot overlap mid-point is what needs to be referenced to the horizon. This is marked in the 1st image below. First you simply loosen the white nylon screw on the ADC which allows you to turn both the white nylon screw & the slip ring until they correspond to that slot overlap mid-point & then lock the white screw & slip ring in that position.
After doing that & moving both levers to this mid-point also, you place or position it into the filter wheel using whatever connector you have with that white screw etc pointing either left or right & horizontal/level to the horizon in an SCT, also depending upon whether you have a "lefty" or "righty" ADC.
SCT's are easy in that all you need to do is every now & then check to see that the white screw is maintaining a level position as the mount & scope moves over time, correcting the entire ADC unit's position when necessary by rotating the entire ADC body so that the white nylon screw stays level with the horizon - it is NOT the screw or slip ring that gets moved, it merely serves as a reference point for the entire ADC body after your initial fixing of them at the slots' mid-point overlap. You of course alter the position of each lever to compensate for dispersion appropriately.
It can be deduced from the preceding that you really need to rotate the ADC wherever it is situated in your train independently of any other aspect of the imaging train: in the pikky above I have 2 of the top elements (one at each end of the ADC, that's the right hand fitting in next pikky) & my EFW has a fitting like the left-hand fitting in this 2nd pikky: I can then insert the nosepiece of the ADC into that 2nd "T2(male) -1.25" (female) holder" which then screws on to the EFW.
By loosening the additional "T2(male) -1.25" (female) holder" clamps I can rotate the the ADC with it's nosepiece in said holder to keep the white nylon reference screw horizontal & then by loosening the clamps at the other "holder" on the opposite end of the ADC I can compensate the camera's orientation for my re-orienting of the ADC - this is necessary if you want your planet to continue any of its' X & Y corrections on an even keel!
Hope this is clear but don't hesitate to ask for any further clarification!