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Good question @chriscook
Usually, if I want something to be a specific length I will create a “polyline sketch to scale” markup set at the length I’m hunting for. Then, with my regular markup tool I will click on either end of the reference line to establish the desired length. You can then deleted the reference line – or re-use it.
Taking this a step further if I need a series of offsets – say a 5 ft offset from a building pad I will create a series of 5 ft squares using the “rectangle sketch to scale” around the structure and then run my measurements around the corresponding offset corners. This also works well with “ellipse sketch to scale” markups.
I tried creating custom tools using “polyline sketch to scale” but haven’t had much luck them – though I can’t remember what the specific issues were. Might have to try that again…. 🙂