What is the fuel-oil mix ratio on a 1999 Honda CR250?

This would depend on what type of oil you will be using for your premix. The oil base stock is either petroleum, semi-synthetic or synthetic oil and is mixed with gas at a ratio ranging from 16:1 (petroleum) to as high as 100:1 (synthetic).

My 2 stroke I have is a 2001 Kawasaki KDX200 that I run Yamalube 2R at 36:1 with no problems.

Although synthetics say you can mix them at 100:1 I would not, that seems way too lean to me.