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That subtension as it is called may be different among brands of scopes so there is no standard measurement. One way you can determine is to place a grid target at 50m for example and see the span from centre to post. If for example the reticle portion spans 6" then you know at 100m it will span or subtend 12" as it is twice the range. At 4X I suggest 50m to enable better resolution of the target.