Borderline stuff really? So borderline that Mr R suggests hitting 10,2 v 4, but keeping to BS and standing on 9,3 v 4 or 8,4,v 4 - the probabilities balance on the fact that there is one additional ten card in the shoe and one less small card and 8 or 9 (which if drawn will turn a 12 into a probable winner) between the different hand compositions. As it's nearly all 6 deck in the UK I haven't looked at the effect of hitting / not hitting on single and double deck games.
Assuming OTT of a new shoe and just one player, if one hits a total of 12 there's virtually 4 chances in 13 of busting but 9 chances in 13 of improving the hand or it remaining a stiff - no brainer really. When I started playing I was surprised that BS dictated standing on a 12 against a 4,5,6, but I now appreciate that this is due to the fact that with a dealer 4 and up there's a greater probability of winning by the dealer busting out than through drawing an advantageous card if hitting. The 10,2 v 4 hand is the borderline bit.
A bit like when do you stand on a 12 v 2 if counting cards? HiLo +3 or +4 true? And of course the cards that will turn a 12 into a winner or probable winner aren't counted at all . . .