Well, you can make it so concentrated that you don't add any water at all! You can just feed them straight sugar, but this is probably best done in the middle of a cold winter, rather than a dry, hot summer. The bees get water from the syrup as well.
When adding water, you would have a very hard time getting more than a 2:1 ratio, so this is typically as concentrated as you get.
As a side note (well, question), in your area should you be starting a small, weak swarm as a new hive at this point?