Help me out. here are the basics

I run 1/3 glow at 10/10 and 2/3 gas

To make 15/15 into 10/10 I need to dilute it by 1/3.
15 is 33% larger than 10

Take 9 ounces of 15/15. 1/3 of 9 is 3.
So adding 3 ounces of methonal to the 9 ounces of 15/15 should give us 12 ounces of 10/10

Am I right so far?