Hi, you posted in the right place! Molasses labeling can be confusing, because what one brand means by "full flavor" may not be what another brand means, as you discovered. It's more important to know if you are using a first, second, or third molasses, which refers to how many times it was boiled. (You can read more about the differences between grades of molasses here: http://healthyeating.sfgate.co...s-molasses-2894.html)
My recipe is written to use a first or "mild" or "light" molasses. It sounds like you inadvertently bought a third or blackstrap molasses, which has been boiled longer than light molasses and thus has a less sweet, more intense flavor.
I suggest you stick with a mild or light molasses (other US brands of molasses should work fine, as long as you use the right grade) rather than try to thin dark molasses with corn syrup, as the latter has a higher water content, which can lead to more spreading and cracking if you don't get the ratio exactly right.
I couldn't even hazard a guess as to the right ratio unless I re-tested the recipe with the exact molasses you used. Though you could try gradually adding corn syrup to the molasses you have until you have a flavor you like and then add the same amount of the mixture as I call for in my recipe, and see what happens. If the dough spreads too much, but you like the flavor, then add more flour or reduce the leavening. This is how I would begin to re-test the recipe if I had no choice but to work with the dark molasses.