I would defer to the equipment manufacturer's instructions. It may depend somewhat on the technology. For my photo sensor type chrono it states a minimum of 10 ft between the muzzle and sensor for high velocity rounds and at least 5 feet for slower velocity rounds. For practicle distances like 10ft to 15ft, its not going to make any difference in measurements. At matches I've shot that use other types of sensors, they still ran about 10 to 15 ft between the muzzle and the first sensor. The second sensor was another 3 to 5 feet farther away.
Biggest factor is the measurement statistics and how you load to meet PF. As a general rule, my loading velocty is the required velocity plus 2 time the measured standard deviation (there is a good reason its not 1 or 1.5). This gives my a high degree of confidence that even with unknown factors, I'll be able to pass. Kinda drifting off the main topic but thought it was worth mentioning.