I have used my range finder on moving objects, BUT they were no where near as fast as even a park flyer. The issue with a range finder is it will tell you the distance from you to the object you are tracking, It will not give you the distance above the ground. BUT if you know the distance from you to the object and the angle you are sighting at it is simple math to determine the altitude. a X Sine C = c, or in our case, Distance from you to plane X sine of angle = altitude.
The range finder that I have is a Leopold, it will tell you both the distance and the angle at the moment you push the button. This feature is used by bow hunters to determine the ground distance to the target when hunting from a tree stand. Unfortunately its built in math will only calculate the ground distance to the object but not the altitude. Any smart phone should have no problem doing the math.
Another way if you are using a range finder that will do the math for ground distance then, Distance from you to plane˛ - Ground distance to plane˛ = Altitude˛, then take the square root of Altitude˛. Again any smart phone can do this math.
As far as waiting for the laser beam to get to the plane and back, the laser beam is traveling at around 186,000 miles per second, it takes about 4 seconds to go to the moon and back, or it can go around the world about 7.5 times per second. It is really fast, 400 feet is nearly instant. It will take that laser 0.003 seconds to get to the plane and back to the range finder. in that time if the plane is traveling at 200 mph the plane will travel just under .7/16 of an inch (0.432 inch).