I think that is about equivalent to a "bump", which is close to a
"plum"--these terms are usually applied only in the negative sense,
though, as in "a 'bump' too far" or "'plum' out of line." "Plum", in
this regard, is much like "way", but indicates a much smaller amount;
however, a 'plum' is larger than a 'bump'. Any etymological
association with "tad" is only speculative.

Dan