We still recommend measuring the distance to your target from the front of the lens. This is the same plane of reference we recommend when measuring with the fixed focal length lenses.
There are definitely factors which could compromise accuracy when using the Zoom Lens. These include but may not be limited to the inability to exactly determine the focal length setting, the length of the lens, the depth of field, and the sharpness of the focus.
At 3', a 1" difference in the distance to the target results in a 3% error. This error is further reduced at 10' so that a 1" error in the distance results in a 1% error. These differences are relatively minor when compared to measurements taken with a vibration sensor which typically has a tolerance of +/-10%.
We recommend consistency and as much accuracy as possible, but the most powerful use of the software is to convey the message of the motion. Motion Amplification will amplify and show the motion regardless of the lens used or the distance to the target.