Your best option is to have a bike fit done if you haven't. AS I'm sure that you are aware, there are many factors to being comfortable on the bike.. flexibility, stem, top tube and head tube lengths, frame geometry etc... A fitter can actually look at you while you're on the bike, and give you much more feedback then we can online.
With that said, there is a very vague measurement that you can try. Sit on your bike and put your hands in the drops of your handlebars. Once you do that, look down through the stem to your front wheel... in an ideal world you should not be able to see the hub of your wheel. That distance can change quickly change a bit as we move back and forth on the saddle, but as I said earlier, this is a very basic measurement.
Good luck !!