Finger In The Air Estimate

A measure of a value based on an unreliable or inexact metric, e.g., an estimate of wind speed based on its cooling effect on one's wet finger.

Alternatively, this evokes images of a particular finger in the air, like the response you would give a marketoid after asking for the 5th time that day "When are you going to have the fully featured product done, for which I only saw a rough prototype?"