I suppose wheel building techniques differ greatly from one person to the next. But measuring effective rim diameter (ERD, the span across a rim) is an important element in wheel building in my opinion. My understanding is that the ERD can be different simply from one rim to the next of identical brand and model for various reasons.
How I measure ERD is I accurately cut a pair of spokes to 200mm(this cut removes the elbow section, length from the threads down, I use 100mm lengths for smaller wheels) and then tape(http://www.amazon.com/gp/product/B001E5ZWT4/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B001E5ZWT4&linkCode=as2&tag=xbnijgbr-20&linkId=W2KBASRSEXHJQ4ZE tape I used) a rubber band to each end of the non threaded side. I put the threaded sides through the opposing rim spoke holes with nipples screwed into the depth I see fit. I measure the gap between the ends of the spokes and then add 400 to this number. A great tool for accurately measuring the cut spokes and the gap between ends of the spokes when measuring ERD is one of these http://www.amazon.com/gp/product/B000EJUBBU/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B000EJUBBU&linkCode=as2&tag=xbnijgbr-20&linkId=FEOLOKZJ5M4YXMOT , this is the particular one I use. Great tool to have for many things. I suggest measuring the ERD 3 times and using the average of that number.
You might be able to get your friend to calculate the spoke length for you, or maybe he can just answer your questions as well, I'd ask him either way. He might show you have exactly he measures ERD.
I could be wrong in thinking it's important to accurately measure the ERD for every rim, but I am a stickler for detail. "The devils in the details." I imagine that the detail of ERD might be as important as entering the correct flange spacing measurements. Everything sort of hangs together when calculating spoke length. "The only difference which makes no difference is no difference."
One thing others may have mentioned but I missed, there are spoke thicknesses of many varieties. You probably want to get single butted 13 gauge(2.3mm)/14 gauge(2mm) spokes, which means 13 gauge (thicker) at the elbow and 14 gauge(14g) beyond the elbow and a little bit thinner. Some think it's better to go thicker, spokes need to stretch under tension to retain their tension, the stretching helps preventing the nipples from turning when you go over hard bumps as you travel. A thicker spoke will need more tension to stretch enough for this effect than a thinner spoke will, a bicycle rim probably cannot handle the tension needed to appropriately stretch some like straight 13g and thicker spokes, this is why we would probably use 13g/14g spokes or thinner. The best value for you is probably 'sapim strong' (sapim is a brand, strong is a product) spokes with sapim polyax brass nipples.
In summation, there are a lot of little details that go into wheelbuilding, more than just calculating spoke length. If you are going to indeed hand off the unlaced components to someone else then it's probably not as important that you seriously take some time to read a few books on the subject, but it certainly wouldn't hurt.
http://www.sheldonbrown.com/wheelbuild.html
http://www.wheelpro.co.uk/
Might be good sources of knowledge on things for the process.