ISA Temp is +15 Centigrade,. .ISA lapse rate is 2 degrees Centigrade (1.98 for the purists) per thousand feet.
So take your altitude times 2 (for 2 degrees C.), minus the +15 degrees, which is the standard day at sea level.
Using somebody else's example for 30,000 feet: 30X2=60-15=45 degrees. Numbers of degree(s) on either side is your deviation from ISA. I think the exact number was 44.6 degrees, probably using 1.98 instead of 2 degrees. I can live with that error, can you? Quick and easy.
Enjoy.