I personally use a set of dial calipers. There's a depth gauge on the end opposite the jaws that you can use to measure each side of the site down to .001 accuracy. Better depending on your calipers. But basically you measure the width of your slide and write it down, then measure the width of your sites and write it down. Subtract the two values and divide by two. Then you'll know how much space should be on each side.
Please post your thoughts on the sights as well. I've considered those but not sure yet.
I usually use a ruler and measure where the center of the stock rear sight an mark a little line on the slide with a pencil, and measure the new rear sight an put a line on it with the pencil, then it's perfectly centered when installed! That's how i do it..