[Varyon]

I'd do it by rounding the delta - either rounding down, or rounding normally. That is, 30% of 18 is 5.4, which rounds down to 5, so you can do +5 (for 13-23). This will give the range the same sort of symmetry as is seen at a base of 10 (or 20, 30, etc), rather than having some cases be lopsided (like 18, which can be -6 or +5). That's just a personal preference for my slight OCD, however.