Method 1:

To measure a dog collar accurately, use a comfortable dog collar as a reference point and measure from the base of the buckle to the hole you typically used. This hole will become your new center point.

Method 2: 

Use a soft measuring tape. Measure around the dogs neck where your collar sits comfortably and at proper tightness.

Method 3: 

Measure using a rope or string around your around the dogs neck where the collar sits comfortably and at proper tightness. Then measure the rope or string with a tape measure.