To get 2.5 volumes of CO2 into a beer that has sat about 65 F, you will need about 4.5 ozs of table sugar.
3.6 ozs of table sugar = 50 LEVEL teaspoons of sugar = 1 LEVEL teaspoon of sugar per 12ozs bottle.
You have already added 1.5 ozs of sugar. When you open the bottle, you will loose a small amount of CO2, but most will be entrained in the beer. Adding a half LEVEL teaspoon of sugar straight to the bottle should bring you up to somewhere around 2.2 volumes.
To get to 3 volumes (which is getting to the limit where they guarantee the bottles are safe), you need to add a total of 4.5 ozs to 5 gallons; or in your case, an additional 3 ozs of sugar. 3 ozs = 40 LEVEL teaspoons, or about 4/5ths of a teaspoon to each bottle