22% oil is about all you want in there. At 17% there is 22 oz. of oil in the fuel. If you want to go to 22% oil content it's 22% of 128oz which gives you a total of 28 oz. So, add another 6oz of oil to get the total of 22% total oil.
Actually, this is not totally accurate because it leaves the nitro out of the equation.
Clarence Lee generally recommends 22% and it should, as you know, always include some castor since castor burns at a higher temperature than synthetic oil and protects against a lean run.
There is a better equation than this which is more accurate but this works fine. Just be consistent when you whip up your fuel each time. You may even want a couple of ounces less oil since it can and does get pretty messy.

3dbob