This is a very old thread, but the Wiki page https://webwork.maa.org/wiki/AdaptiveParameters still had (until today) the old code which would lead to the same issues which Danny had when naively modified for the case where the correct answer is a scalar multiple of a standard answer.
I have edited the Wiki page to provide a better standard recipe considering this forum thread and that at https://webwork.maa.org/moodle/mod/forum/discuss.php?d=324 where the proposed approach would also suffer the issues discussed here.