Before you start measuring and drilling.....have you considered this and have you tried, in flat water, and in steady winds, to steer the Hobie with rudders UP?...(Bronze IV sailors will recognize this challenge).
Using more jib to bear off, and more main to head up, and body positioning/weight, one can steer the Hobie rather well. It also teaches one the boat's tendencies. In an ideal world, the boat will have almost neutral helm, with a slight tendency to head up into the wind. As you know, this is a safety feature.
My 18SX for example, is mostly neutral, until the wind reaches higher than about 8 knots. Then weather helm begins to kick in, light, but it's there. It is also possible that I am over-sheeting the main, so I'm adding tell tales this week to aid my 'measurements'.
Measure up 48" from the base of the mast, make a mark. From that mark, measure to both clevis pins at the bridles. Should be the same, at between 103" - 107". That sets your rake to an optimum position. Too little or too much rake, as you know, can affect how the Hobie performs (in terms of lee helm or weather helm).
Search the Forum re setting rudder rake...I remember readying about this...a long time ago. Enjoy your H18, a wonderful craft.
1989 Hobie SX18 Sail # 1947
'Only two things are infinite, the universe, and human stupidity. But I'm not sure about the former.'