Default Groundspeed in P3DV4 Flight Planner

I've just switched to P3DV4 and I'm trying to set up some scenarios.

In P3DV3 the default groundspeed for a flight planned AI track was 250 knots. In P3DV4 it is now 1085 so any AI target runs away at a great rate of knots! Selecting the "NavLog" box on the flight planner screen shows the flight plan parameters and there is a value for estimated GS and a blank box for actual GS. The figure for estimated GS seems fixed and can't be amended. Is there a way to do this within P3D?

The Learning Centre says:

"GS (Kts): Each row in the GS (or ground speed) column is split into two cells: The top cell indicates the estimated ground speed, which is based on the speed of the aircraft selected when the flight plan was created; the bottom cell indicates the actual ground speed, which is calculated and updated during the flight."

I can see no way to set the speed of the aircraft when creating the flight plan.

Oliver from Lorby SI has suggested setting up new waypoint lists but struggling to achieve that so far.