That may not be the case when you're:
Originally Posted by PeterB
a) shooting through the window, which will add its own glare and reflections (and which seems to be the plan unless I'm misreading)
b) have the step wedge out in the sun as well, which would add glare and reflections off the face of the step wedge.
If you're shooting with the target in the sun, you may want to use a reflective step wedge. If it doesn't cover the range you need in one go, you can make bracketed shots to introduce the wanted greater or lesser densities.
I'm not really trying to be difficult, it's just that there are contingencies that we don't always take into account, and it's best (and in the long run easiest) to eliminate error up front rather than puzzling over where it came from later.