Depends on how accurate you need?
Bryan Litz tested many of the commercially available Chronys on the market. In order to use the optical sensing type, you should be perfectly square to the gates and try and a use longer gate spacing to minimize errors in gate alignment (parallel). It was determined that that a small 1/8" difference in sensor spacing leads to a 31.3fps error in velocity for standard 12" units. Magnetospeeds were ranked near the top of the pile next to his own custom made 4' Ohler and artificial light source. I am currently saving my pennies for a magnetospeed.
If you want to read more about this, get his latest book "modern advancements in long range shooting", an excellent read!