Grab yourself a whip (even just a stick will suffice) and place the end of it against the bottom of the saddle flap, so that it is parallel to the stirrup leather. Where the leather folds at the stirrup, place your fingers in line with this point on the whip and keep them there.
Go around to the other side, keeping your fingers at that point on the whip and do the same measurement from the same location on the flap. If they're even, your fingers should be at the fold of the leathers.
