How much skin do I pinch when using body fat calipers?

classicalguitar1

In other words, how much gap should there be between my thumb and index finger when pinching my skin to use skin fold calipers.
I have heard the phrase "pinch an inch" lots of times but is this true for body fat caliper testing.
The reason I ask is because I can take several readings at the same body area and depending on how big the gap between my thumb and index finger is depends on what the measurement is, hence I always get difference readings.
So someone please tell me in the interest of accuracy exactly what I should be doing.
Thanks
Dan