I'm sure everyone has their own way of testing things but after I checked the window frame I found several equally reflective and staggered laser targets, in this round bales.
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By lasing edges and tops I can see exactly where the beam is getting a return from. Although things seemed bang on at 100 meters (0.7 mil/2.75" high) past 400 meters I had to hold 0.4 to 0.5 under instead of the expected 0.1 to 0.2 and so I adjusted it further so it's around 0.2 high (need a 0.2 hold under) at 400m. Also had to slightly adjust windage.
Again I'm sure everyone has their own way of checking things but this has worked for me.