Need help balancing tonearm/ heavy cartridge

I just bought an Audio-Technica AT150ANV cartridge for my SL1200mk2/KAB turntable. I want to use it on my Sumiko HS12 headshell. I mounted the new cart last night and could not get the arm to balance, even with the 10.5 gram accessory counterweight in the back of the arm.

The HS12 weighs 11.5 grams and the AT150ANV 9.5 grams.

Can anyone offer a solution to this problem?
If the arm still won't balance with the extra aux
counterweight, about your only option is to find a lighter
headshell, (or a lighter cartridge, but the 150ANV is a
sweet cart).

I looked at the KAB aux counterweight, (I'm assuming that is
the one you are using). If the threaded stud is long
enough, you could try adding a washer to it. It would need
to be thin, but large enough to add the weight.
If I understand the problem correctly, the solution might be easy. The only important thing is the tracking force. Assuming that you are using a scale to set the vtf, it is not necessary to balance first. Balancing is only important if you are using the arm's built-in scale to set vtf. As long as the rear weight difference is less than or equal to the tracking force, just use your scale.

Tack on some additional weight, BluTak or putty of some sort should work.
As Mofi says, get a lighter headshell. the ADC Magnesium shells are light, as are the vintage audio technica (e.g. ls9) and sony ones (I see an 8gr one on the bay right now). Also, some of the Yamamoto wood headshells are light (e.g. the boxwood one is 8gr). Maybe also change your screws to alu ones if you're not off by much.
You could also try mapleshades tone arm resonance kit.