You may want to include a “default” value for when calc is not supported.
I am doing this currently, but as I’m sure you can imagine it’s hardly a fully-functional option with gutters. I cannot, for example, have a 50% column with
width: 50% because well… margins will cause that to break. So I have to attempt to find a magic number that is ‘close enough’.
What I might try and do is for IE8 switch to a % based margin so that the magic numbers are easier to configure. Or just remove the margins as I mentioned before. They add a level of clarity to the site but not so much that their absence would ruin the experience.