I only file flat at the range just to get the height right and I do use the tape!
Then when I get home and can sit at my bench under more controlled conditions, I take out another ruger and place it in front of the one I'm working on to use as my guide.
Yes, Doc, clamp the revolver poiinting away from you so you can use both hands to SQUARELY draw file the sight to the right contour with a fine file. Constantly review your progress by looking at it from the side and comparing it to the other gun (or your plastic reproduction).
Once satisfied with the results, tightly wrap your file with 1000 grit paper. Continue to draw file with the paper until all file marks are gone. Before moving on, pick up the gun and sight at a bright wall as if your aiming at a target. If the sight is not square and flat on top, it'll show up now!
You can cold blue at this stage. Or if like me, you'll take a dremel tool with a hard flat surface felt buffing wheel and gently polish with white stainless steel rouge until it's mirror bright. It will take and hold the bluing much better with a high polish. Careful not to round the edges by pressing too hard with the wheel.
Clean with alcohol and use only Brownell's Ox-Pho bluing cream. If you holster the gun, it will eventually wear off just like the factory blue. Just touch it up once in awhile. A black indelible pen over the blue will give a more durable flat black finish and is easier to touch up.
It's worth the effort to get it right. People won't believe you ever filed the sight.