In fact, it is so easy to miss a tooth. It was actually easier to install the belt with an extra tooth on the top section. The correct fit is a very tight, precious fit, particularly with a new belt. So if you used no marks, it is recommended that you re-count the teeth to make sure. By now even if you have the marks, they have shifted. But the tooth count is the ultimate and there is no direction as said before.
If you don't want to do a tooth count, you can try lining-up the 3 sprockets in the up position. If they line up perfectly, you already have the right tooth count.