Tennessee Volunteer is right, +4 if you are using Hi-Lo (the most common system used, particularly in shoe games--6 or 8 deckers). Also, if the game offers surrender, surrender on 0 or more. Thus, you would never stand on a hard 15 if the surrender rule is offered.
I believe you can get the Hi-Lo index numbers for the I18 from the Blackjack Tutorial offered through this website (lesson 10 or 11 or thereabouts). Included in the I18 is the index/strategy variation you mention, stand on a hard 15. (I18 are the 18 best strategy variations from basic strategy and account for about 75% of all gain available by changing from basic strategy when warrented. Hard 15 versus 10 is number 5 on the list, with insurance, 16 v 10, and splitting 10s versus 6 & 5 taking the top four spots.)
Good luck.
