How about this:

1) Roll d20.

* If you roll 1-5, keep this total and roll again, subtracting the "tens"
digit of the second roll from your total.

* If you roll 16-20, keep this total and roll again, adding the "tens"
digit of the second roll to your total.

If on the second roll you get 1-5 or 16-20, add/subtract the "tens" digit
and roll again. Repeat as necessary.

    John Harper         http://www.shootingiron.com/feng
    ----------------------------------------------------
    Talislanta RPG Website     http://www.talislanta.com
    Shootingiron Design      http://www.shootingiron.com
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