PR, how is your rail bolted on? What types of bolts is it using? If it uses flat socket bolts (triangular shaped if viewed from side and mount flush to surface inside a countersink) it should center itself up but if it uses flat socket (normal bolts usually with knurled heads) there will be some play.
I do agree a milled on rail would be best. Perhaps even a high rise or an option for one if the material is large enough to accommodate it. I still am deciding on my rail, i need to get one ordered.
I recommend some lube on the 12g or inside the cap to make tightening it down easier. I have a small tin of hater sauce i plan to keep on me to dip and end in every few 12g's.