The 12 inches is regarding the UVB light itself. The temperature will depend on what the temps in the room iitself is. The only way to tell is to place the probe of a thermometer in the basking area and see what the temperature reads
I have an SB 100 set at 16", in addition I needed 150 CHE and a 90watt halogen flood to get my temps in the low-mid 90's...it took a lot of playing around to get it right. Make sure whatever you use you have a heavy wire shield to keep your ig from touching the bulb, they don't know they are getting burned until after it way to late.