So much depends on your atomizer, if it can't wick enough juice and supply enough air, when you turn up the volts/watts to high you are going to get dry nasty hits, really nasty. Also your mix and nic level is going to play it's part. Personally, I find high PG juice harsh and makes me cough so I mix at 80% VG. We all have to find our preferences, juice VG/PG ratio, nic mg level, type of atomizer, and personal vaporizer. And as one starts to dial in one aspect the other might change. What kind of atomizer are you using? If it's just a cheap generic clearomizer, I'd recommend trying the Aspire Vivi Nova BDC tank. The Aspire will deliver great vape and be able to take whatever voltage an ego twist can put out. Once you have a decent atomizer, you'll be better able to discover what juice mixes you like best, without having to deal with dry hits. Then once you discover you preference, then you can focus on better gear if you want to make an investment into vaping.
Sweet spot can't be determined until you can vape your juice without getting dry hits. Some juice do taste better at different watts/volts, but then again, I find my sweet spot is around 30 watts. Like I said, so much depends on your atomizer, and whether it can deliver the goods.