You are in warmer than ISA air so your altimeter will be under reading, this makes your true altitude 15,000ft x 1.5 x 4% = 900ft
So you are really flying at 15,900ft, but as you have to be at 15,000ft to be at the minimum recommended flight altitude you can fly that 900ft lower than the stated 15,900ft which will give you an indicated altitude of 14,100ft but in reality you are at 15,000ft, only convert from a FL if they specify you are at a FL