Extend layout field for n/5 or n/10 columns

I have a page with only 10 columns and can’t use the 6 or 12 column grid settings in the layout field. Can I extend it so it allows 1/5, 4/5, 9/10, etc.?

Thanks for any guidance.

Maybe for now I’ll just use the existing base 12 layout system but make the conversion to base 10 in my CSS.