Another rule of thumb
The one I use is that the altimeter will be off by 4% for every 10º off ISA. Obviously if it's cold the altimeter will overread (ie it will be "dangerous") and if it's warm it will be "conservative" (ie underread).
So for a simple example if the published OM crossing height is 1000' at a sea level airport, and the OAT is -5° (ISA minus 20), you can expect the altimeter to overread by 8% (so 1080' at the OM.)
MD