The 11GPM sounds pretty high for a static pressure of only 50psi. I don't question that your hose bib can't fill an 11 gallon bucket in 1 minute, but if you design for an 11gpm system, I think your going to have an inadiquate design.
A quick look at one of those "short cut" guides indicates that for a 5/8 meter, 3/4 copper water line, and 50psi static pressure, you shouldn't design for more than 6GPM.
However, if you tie directly into the water line just after the meter you might find that you've got more than just a 3/4" copper line. In my case, I found that my water line from the meter to the house was a 1" PVC. The PVC then converts to 3/4" copper just before it enters the basement (building codes prohibit PVB for exposed pipes). Should you find this to be the case, you'll be able to design your system for 8GPM (or a little more) and won't have much of a pressure drop in the house when the irrigation system is running.
Additionally, tying in to the water line as close to the water meter as possible will help reduce your pressure losses. You might have 50psi static pressure, but once water starts flowing through those pipe, friction losses will eat away at that pressure. The more pipe the water has to run through, the more the presure losses. And starting with only 50psi doesn't give you much breathing room. When the final system gets up and running, your going to lose 2-3psi through the meter, about 3-5psi through control valve, you're going to want at least 20 (and more like 30) psi at the sprikler heads, so that leaves as little as 12psi of pressure loss available for friction losses and through the BackFlow. In the range of 5-8GPM flow rate, you'll lose 1-1.5psi for every 100' of 1" pipe PVC pipe (3-4 psi if you use 3/4" PVC). The backflow can likely eat up another 3-5psi. And if your area requires an RPZ for backflow, that's going to be about a 15psi pressure loss.
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