Best way to measure is to park on level ground, get a ladder, a straight 8'
2x4 (or something else long and straight), a level and a tape measure.

Place one end of the 2x4 at the highest point on the roof (or take several
different measurements), level the 2x4 and measure to the ground from the
bottom of it.  For a Westy it's the top of the skylight.

Add 2" for safe clearance.  So, if you measure 6' 8" you need 6'  10"
clearance.

Find a label maker and put this on the corner of your rear view mirror!

Years ago I saw a guy in downtown Seattle try to go into a parking garage in
an Adventurewagen.  Pretty much destroyed it, but I think Ed was still in
business back then so he could have it replaced.

Stuart