The correction is the same whether it is colder or hotter than ISA, 4% change in altitude for every 10ºC deviation from ISA. In ISA+ temps the altimeter will under-read (safe error) and in ISA- temps the altimeter will over-read (unsafe error).
In your scenario the temp is ISA+23.
4% for every 10º deviation so,
4 * 2.3 = 9.2
So if you're on slope at the OM the altimeter should read 9% less than the published height or about 1090'
To make things a bit simpler use 5% for every 10º deviation or 10% for 20º deviation. and round to the nearest 10. That would give you a correction of 120' in your example.