I have this, but I feel 4 is too big for my sidebar width and 3 is too small (it has to add up to 12).
-
You could certainly create your own classes:
.col-md-3point5 {width: 28.75%}
.col-md-8point5 {width: 81.25%;}
I'd do this before I'd mess with the default columns. You may want to use those inside these.
You'd probably also want to put those inside a media query statement so that they only apply for larger-than-mobile screen sizes.