I'd do a quick trial on my own with a 20mm packer, once around the island you'll know if it's right or not. If not, do the same with a 10mm packer. Once you are happy on your own, try it with the rest of the family. It's trail and error and a compromise as to what height the OB should be, but you will hear and feel when it's right.
However, it's the first weekend out with the kids, so it really doesn't matter. The most important thing is that you and the family have a good time and enjoy it. But, do watch your speed! especially on the North side (road side) of the lake. If you do fancy a bit of speed only do it around Howtown and watch out for the ranger.
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