Thanks every one, for your comments particularly Rockin Robbins. This is a very interesting subject.
A point Rockin. You suggested, I think, that the formula is invalidated, or is dependant upon a particular screen resolutuion "...because the angle subtended by one division is dependent on your screen resolution". But surely screen resolution is irrelevant. If one knows the angular value of each division of the reticule then the angle subtended will always be the same.
My practice is to use the tangent rather than the cosec, this gives the range to the target at the waterline rather than to the masthead. My reasoning is that although, at longer ranges, the difference is small and can be accepted at shorter ranges i.e below 1000yds the error can be significant. I like to get in to c750 yds.
Joe S. I agree with you that trying to get a range using the stadimeter especially with 'no stabilised view' selected is a nightmare. Howver, why shouldn't you use the stack! If you look at the manual it is possible to estimate the height of the stack or any other part of the target, bridge, derrick et al. then do the trig calculation to get the range. Be aware though that because you are using a smaller height the error in range will be greater. On a pitch black night with rain though, it might be better than nothing.
Incidently I use an old Thorton PP221 slide rule fo my calculations.
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