Cutting Windshield Height
For most circumstances, riders prefer to look over, not through the windshield. There is a delicate balance between the height below which there is too much wind, and above which the top of the windshield seems directly in the rider's line of sight.
Finding the correct height
The easy way to find that "tipping point" is to put a few yellow sticky tabs (aka, "post-its") and move them up and down on the inside of the windshield until the perfect level is established. At that point the yellow tabs should be as high as they can be without interfering with the rider's line of sight under most conditions. Keep in mind two things: 1) You will sit lower in the saddle after riding for a while, and 2) If you cut off too much windshield, you can't put it back.
Getting started
Once you know how much windshield to remove, measure the distance from the top of the glass. Find a big piece of thin cardboard and trace the top of the original windshield on the cardboard. Then cut the cardboard along the trace line and set it aside.
Preparing to cut
Use blue painter's tape from Lowe's or Home Depot and tape the entire upper part of the windshield, inside and out. Lay the cardboard template over the windshield and move it down the distance that you previously measured. Trace the shape of the template on the blue tape, moving the template slightly left and slightly right as necessary to be sure that the lines on the side match the windshield at its lower levels.
Cutting
You can actually cut the windshield on the bike. My assessment is that it was easier to cut it on the bike than to try and secure it off the bike, but that was because I had nobody to help at the time when the urge struck me to undertake the task. Regardless, I used a DeWalt cordless jigsaw with a fine tooth blade and sawed up from one side and across, turning up midway to discard the first half. If you have a helper, it is probably just as well to saw straight across-- makes it easier to keep a straight line.
Filing, sanding
I used a file and sandpaper to smooth off the saw lines, but really the cut was not all that rough to begin with. Nevertheless, being somewhat impatient, and anxious to get out and ride on that sunny day, I also took a shortcut and used my DeWalt orbital sander to speed up the process. Do the rough work with the tape still on, and then remove it to do the fine stuff. Round the edges just slightly, using the finest grain of sandpaper on a firm sanding block.
Final finish
Buy the smallest can of Methyl Ethyl Ketone (MEK) that you can find at Lowe's or Home Depot. Keep in mind that this stuff melts plastic and a lot of other stuff, so it's not a good idea to get it on your fingers. Put on some chemical safe rubber gloves, and then take a soft cotten cloth (a piece of old t-shirt will work fine) and wet a small spot of the cloth with the MEK. Be sure that there is not enough MEK on the cloth to leave any chance of dripping on the windshield or anything else. Carefully wipe the MEK dampened cloth along the the sawed/sanded edge of the windshield to leave a factory smooth finish.
Harley Davidson Road King Classic with sawed down windshield.