I had to perform some detailed calculations to install the gas strut brackets and braces on the hatch today.
I weighed and measured the hatch. According to online sources, a strut should be about half the length of the hatch. Using statics, I calculated two 20 inch long, 50 to 60 pound struts would be needed. I bought two 20 inch, 60 pound struts and brackets from Albuquerque's Central Trailer Supply.
After observing my Subaru wagon's struts, I used Sketchup Make to check strut layouts and locate best mounting points. The diagram shows how I determined the best mounting points.
My struts were 20" long extended, and 11-3/4" compressed all the way.
Point A was almost 20" from hinge point and three inches inside hatch skin.
With hatch up, I measured an arc with 20" radius from point A.
With hatch down, I measured a second arc from point A with 12-1/4" radius so strut was almost but not fully compressed.
Where the two arcs intersected is where to locate point B, second strut point.
On each side of the hatch, I installed strut brackets and 1x2 poplar braces for extra strength. Next, my wife held up the hatch and I struck the first arc, closed the hatch and struck the second arc. Point B was where the two arcs met. I installed a second set of braces and brackets on the side wall interiors.
I will install gas struts after the hatch hinge is installed.
Next skin the hatch!