Hi, Andy, and welcome.
Pertaining to T206s, I would likely just keep PSA's and SGC's scales on the same level, as you'll see variations within a grade with both companies. One of the few noticeable differences I see between the two is SGC may be a little more lenient on corner rounding (ie. you will see some SGC 50s with rounded corners typical of a PSA 3).
SGC also seems to be far tougher on minor paper loss, or they simply do a better job of catching it -- I've seen PSA 4s and 5s with minor paper loss on the reverse, where this would've automatically knocked it down to an SGC 20 had they graded it.
But overall, they can be pretty consistent, and I wouldn't necessarily bother with adjusting the grading scales to match perfectly -- there are too many factors to consider, and it all comes down to a grader's opinion, and then our opinion of his opinion.
