Welcome to the forums @vfaulkner! Other people have requested this same feature and we are working on making it easier to do custom grid options for smaller devices.
In the meantime there is a fully-customizable way of doing this using Floats and Percentage Widths. Check out the link below. You can call it a work-around for now, even though some may say this is the correct way of laying out your design:
The only thing is that you have to do some math to figure out what percentage width to make all the Div Blocks fit so it adds up to 100% when Floating. Ex: 70% + 10% + 10% + 10% = 100%
Let us know if this helps!