That is a 3:1 might be a little low on some years to keep it full for our part of the world. Usually 5-10:1. Doable though and I can tell your doing your homework. Good news is you will not need a huge outlet system to handle excessive water. What is plan on outlet, suggest siphon system (6")if you have enough fall on backside of dam. Couple with a good wide emergency spillway.

Greg Grimes
www.lakework.com