Unless it a really old mathews bow, the tiller can be measured right to the string. The distance should be the same for top and bottom. Best to start with limb bolts tightened all the way and back down an equal number of turns until you get to your desired weight.
If I remember correctly (I don't measure tiller, and don't think it is important) the tiller on all my mathews bows has been different on the top and bottom. Not by much, but they were different. If you have different measurements and you were wondering if something was wrong, I would not worry about it. All that matters is that your arrows group. Go through the tuning guides, like Eastons, and you can fine tune your adjustments to shoot the best groups your bow is capable of shooting.
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