Started gaming in 1978, played just about everything through to the mid 90's then slowed down to a dozen or so a year. I think I have finally settled on a system. ;)
If its a player character, I simply give them the same CON.
For a monster, I ignore the D&D Con, and give it a CON based on other similar CF/Mythras creatures. That way, conversions remain balanced with existing monsters.