Thehotspot,
If you look at it from a chemistry point of view, 12% plus 9% is 21%; however, since you doubled the volume you would divide by 2. The resulting
solution mixture should be around 10.5% Depending on how the original solutions were mixed you should be close to this number, then just use DHA booster drops to take it to where you want it. I made the mistake of buying a bunch of 6% which no one seemed to want so I mixed it with 9% (also a slow seller) and came up with a 7.5%
solution . I kept a few bottles of that
solution and I add DHA drops to bring up each bottle to where I want it. The formula seems to work pretty well.