If you want a ballpark to start with use 1 ounce of hersheys chocolate syrup to 40z coffee and 4 oz milk or cream. I use skim milk for most of it, but some fat free half and half to give it the right feel in my mouth.
If you don't want to use the hersheys, try using Ghirardelli ground sweet chocolate and cocoa. Use about 1-2 Tablespoons of it.
The flavored creamers are great for adding other flavors with-o oversweetening.
for times when you make a lot of these, or mix up a pitcher full, you can get large containers of Hershey's syrup and pumps to fit them at the institutional size aisle of Walmart or at Sams. Just check the price per ounce to be sure it is less. Most of the pumps I have see dispense one ounce, which is very handy.
For a long time Starbucks employees guessed at the amt of chocolate and other syrups to use - the recipes called for "coating the bottom of the cup" and other eyeball estimations. It is why I always had a favorite barista and quite a few who were not, and why the quality could be so variable. (Not guessing - I had the training by Starbucks). At the Starbucks that finally opened here the recipes are more standardized.
One thing to be aware of is that often the cold drinks need more sweetening than you expect. Hot drinks stay mixed better and the flavored syrups don't mix as well with cold liquids.
If you use hot coffee to mix the chocolate in (even just an equal amount as to the chocolate) and then add the cold milk, more coffee, etc... then you often wind up using less sugar.
