B&G Deckman - Manual (Page 63)

Chapter 3: Navigation
3.36
Current waypoint Meridians from the current waypoint, parallel
lines at equal distance from the current
waypoint.
Projected centre Meridians from the projection centre (the
first point you select), parallel lines at equal
distance from the projection.
Note. The spacing units are in degrees and decimals (ddd.dd).
Hint. For meridian spacing, something like 20° is usually appropriate,
since this gives you 18 meridians 20° apart. The parallel spacing
depends very much on how far apart the two points you choose are – see
below.
You then pick two points and the image is warped so that the great circle
route between the two points is represented as straight and the chart is
altered to accommodate this. You cannot zoom out beyond what is
shown on the chart at the time you select warp image so make sure both
points you wish to select are visible. You can zoom in to position them,
however.