Robert is right - calculation is the best way to get things just perfect if your sweet cocoa lists the percent of sugar or the grams of sugar per gram of sweet cocoa. Sweet cocoa here is "drinking chocolate" and has things like whey powder and milk solids as well as sugar, but the packet I'm holding has 4g sugar per 11.5g of mix, or about one third sugar.
You'd have to check your own packet. That still may not give the relative amount of cocoa unless your sweet cocoa is nothing but sugar and cocoa. That's why a taste test may still be useful unless your packet gives more details on contents than mine does.
(I figure my packet is probably about a third cocoa, a third milk products, and a third sugar.)