A local dealer actually did mine. Cost me $25 and it took him less time to cut the shield than it took me to pay for it. Apparently he had done hundreds, sat me on the bike, asked a few questions and cut. Looks perfect.
actually a Rotozip works best. I have cut many a windshield and even molded a few. I have found that a carbide spiral bit does the best job. Mark the line you want and then cut just above it. The idea of cutting a section from the top and using the factory edge as a guide is a trick I have used as well. I finish the cut on a belt sander to remove any irregularities from it, then just hand sand starting with a 220grit moving up to a 600. You will have a factory edge.
I had mine trimmed by a glass shop. $65.00 and it came out just like it came from the factory. I believe it was cut around 3 to 5 inches. I had to take the wind shield hardware off the windshield first, but it was worth the money to have it done right. If I had tried to do it myself,,,,well I most likely would have been buying another windshield.
As a carpenter, I'm pretty familiar with cutting materials, I'm pretty confident in my abilities. I do like the idea of roughing it with the jigsaw and finishing with the router, so the router bit isn't dragging through the entire width of the cut. I have a rotozip, too, but I think the router has a lot more power and higher rpms, plus it allows a larger diameter bit, further increasing the rotational edge speed.