Hey dude, this is a place to post requests for NEW FORUM SECTIONS. If you are looking for anything DIY scroll down the page a little bit and you will see there are a few sections to ask these questions in. Not scolding, just letting you know so you get the right exposure. You will get great feedback if you pose this question in the right spot.
So to answer your question, there really is no RULE OF THUMB here. It's all about your own personal taste. If I am looking to sweeten up a mix, I use no more than 2 drops of Super Sweet per 10ml. Maybe I'll use some FA Marshmallow instead, it adds a nice mouthfeel as well as sweetening up a mix. Ethyl maltol doesn't exactly sweeten up a whole mix. It will liven up certain flavors in a mix and smooth out other elements of your mix. I think your best bet would be to do some trial and error. Make small batches, maybe 10ml and try one with EM, one with 2 drops of CAP SS, and one with none (ONLY AN EXAMPLE), and see which suits your taste buds best. It's all about crafting what is right for you. I hope that helps.
http://vapingunderground.com/#diy-e-liquid-do-it-yourself.137
^^^ That is where you want to post stuff like this from now on. Welcome to the site.