I've used either a CBST or a Theis (depending on which pistol I carried at the time) for almost 5 years. Currently, a G19 in a CBST is the usual. As I ordinarily wear a jacket of some kind, but it's really not hard to tuck the shirt over the pistol, nor is it very noticeable. On those occasions that I do tuck the shirt, I can wear it all day without any discomfort at about the 3:30 position. It really makes no difference whether I'm sitting, standing, driving, etc.
I swapped the cross-marked belt clips on my CBST with plain ones, just because I thought the distinctive cross screamed "HOLSTER" to anyone who had any clue what they were looking at. The only time that I know I've been "made" was at the NRA convention in 2013 and it happened twice:
(1) I was met pax face-to-face and she picked it out; and
(2) while I was looking at holsters, a holster maker unexpectedly slapped a holster onto my hip to show me how his holster fits. Naturally, he planted it right on top of my pistol. The look in his eyes led me to believe that he was surprised, but neither of us said a word about it. . . . .
1) Put on pants, belt and holster. (The first couple of times you put the holster on, you'll feel like a contortionist. That's OK.)
2) Put gun in holster.
3) Tuck shirt over holster. Let your belt out as far as you can without it actually being undone. Tuck the shirt in all around, being sure to get some fabric between the kydex shell and the beltline of your pants. It may bunch up a little right where the belt clips are, but that's OK.
4) Tighten the belt down.
5) Pull up a little bit of fabric all around on the shirt, so that you have just a little looseness around the waistline. You don't want the shirt so tight that you print every time you move.