Slow Green
Since it is considerably slower, it will be difficult for you to get a sense of distance.
If you trust the figures provided by this web app and putt the ball, you will be able to handle it.

Medium Green
This is the green speed reading for a general golf course.
Green speed in the 9-feet range is the easiest to putt, while 8-feet greens are the slowest among medium green. Believe in the values from this web app and putt it.

Fast Green
This is comparatively faster than the above two.
11 or 12 feet is a speed at professional level. You can rely on the values provided by this web app.

Stride length can be selected in 5cm increments.
When using a laser rangefinder, select 90cm for yard display and 100cm for metric display.
If you have a short stride length, when you use a laser rangefinder and walking measurement, use a large step, or increase the number of steps by about 10%.
*To get an accurate stride length, measure 10 meters and divide by the number of steps.