..this is a work-in-progress...
There are multiple parameters of an XPlane glider you have to tweak to get accurate flight performance, and the most important of these is the characteristics of the airfoil.
In XPlane's Plane Maker (not Airfoil Maker) you specify airfoils for each 'wing' of the aircraft, including the horizontal stabilizer, the fin (aka vertical stabilizer) and the main wing. But the characteristics of the main wing are by far the most important.
Also in XPlane you can move the CG (i.e. the center of mass) forwards/backwards which will affect the trim speeds and the stall behaviour.
... more coming soon...