I patch test at the back of my cheeks near the ears and also going down to my jawline. Depending on product, I might also patch test along the sides of my forehead, and on a small area of my butterfly zone (where I'm most prone to breakouts).
Reason why I want to test directly on my face instead of just behind my ears is because I want to see if it works for my face! Patch testing behind ears can help point out allergic reactions, but if I patch test on my face, I can see if the product actually breaks me out / irritate my skin at areas I normally have reactions.!
I think in your case, it's okay to patch test at where you usually breakout. So you can see if the product aggravates the sensitive areas of your skin and make your breakouts worse.
Of course, remember to start with only little amounts of product and only on a small area!!! Then slowly build up