OK so Tax Guy Bill sent you on the right path.  First determine basis as if there was no life estate.  Often this is the basis when the interest was inherited. Then determine basis of the life estate based on life expectancy.  Us the age of the life tenant at the time of the sale.  Remainderman's basis is what remains.

If the life tenant actually lived there they can qualify for 121 exclusion.  Remaindermen can't.