It depends on the individual and your goals. The higher the dose of test, the more AI you will obviously need. Based off the fact that you are using deca, I am going to assume your goal is to gain mass on this cycle. In this case, I would start with .5mg adex or 12.5mg Arom EOD. If you find that you are staying too dry, you can always reduce it a little. If you find that you are noticing signs of high estrogen, such as holding too much fluid or getting the warning signs of gyno, you can always add more. I have found that each AI works differently for different people. For me, adex seems to be most effective, yet a close friend will only use Arom on his contest prep. Even when I've upped the dose of arom to what I would consider "high", it still wasn't as effective as adex.  The only way to know for sure if you are taking the right amount is to get labs done but this should be a good jumping off point for you