The numerical value of the QNH has no significance in this question. As long as you are flying on QNH, you are measuring with mean sea level as a datum, whether it is 1023, 1013, 993, or any other value.
The only factor is going to be temperature. The column of air between the ground and you is 15ºC warmer than ISA, meaning that it is less dense. Therefore you will need to climb through more of it get a particular millibar change (indicated altitude) indicated on your altimeter. True altitude is going to be higher than indicated altitude.
Using the rule of 4 feet per thousand feet per degree of temperature deviation, you have 15 lots of 1000 feet. so that is 15 x 4, ie, 60 feet per degree centrigrade. The ISA deviation is +15, so multiply that 60 feet by 15, ie, 900 feet.
So, with a true altitude of 15,000 feet, the indicated altitude will be 14,100 feet.
Or, of course, you can use the 'True Altitude' correction function on your CRP5, which I find easier, and teach.