Do not know the percentages but
But the one thing you will know is that if the procedure in that house is for the dealer to turn his first card, the times he instead turns his second card, he will never have a ten or ace in the hole.
So if he turns the second card and it is high, you know he does not have a 20, probably not a 19, but something between 12-18. If he turns the second card and it is low, there is a decebt chance he may not have a stiff at all since his hole card is not a 10. If he turns the second card and it is an ace, he will not have a blackjack, so no matter the count you do not take insurance or even money.
Even with all this knowledge I do not think you can overcome the percentage he gains by cheating in this method so I believe you have two options. Either you move on or call him (or the pit) on it.
About 3-4 years ago I ran into a dealer who did this. I do not think he did it till I put a large bet out. I busted that hand but something registered in my mind that just did not look right. Count rose and I put out a max bet a hand or two later, watching him carefully now because of that earlier hand. I get a 15, he turns the second card, a face. I ask him why he turned his second card and he lies and says he can turn either card but believes he turned the first. I ask for the pit, a moron who can not grasp the concept that if the dealer peaked, turning the second card would give the casino an unfair edge. I tell the pit to get his boss. The boss has the eye run the tape and then instead of just killing the hand does something better. He has the dealer expose his hole card, a four, and lets us play out the hand. The dealer did make a hand but everyone who beat him got paid and everyone who lost got pushed.
The boss then removed the dealer from the table.
Being an unrated "trouble maker" at that point, I asked the boss if the eye could check back over the dealer's complete time at the table because I knew he had done this on other hands. I knew they were not going to do this but the boss came back with comp slips for myself and the other two players on the table, for their best restaurant. I did return to that casino several times since and have not seen that dealer again.
What I can only guess is that the dealer had been cheating the casino with a player/partner and was making up for dumping money to his partner by trying to increase the house take against everyone else. I do not think the casino itself was trying to cheat.