Origin and details of -10 to +10 model of becoming a Christian I've heard of a model for showing how a person can become a Christian: they start at -10 (as far away from God as possible) and gradually work their way to 0 (becoming a Christian) and then, potentially, up to +10 (a fully committed believer).

I'm trying to find more information, specifically:


How this model originated and
If there is any further explanation of the scale, eg "at -8 a person is typically..."


So far all I have found is this page: The Bible For Students which states that the model comes from the Navigators.
It is the Engel Scale. Originally it went from -8 "Awareness of supreme being, no knowledge of Gospel" to +5 "Stewardship" but now it is often shown as going from -10 to +10 (see eg Bill Hybel's Just Walk Across The Room).

A quick Google search will reveal multiple variations, see eg this YWAM variant: