Alright, that makes sense, thanks for the explanation. I do agree the correction is better than just the mean.
I'm not completely convinced about the SD, afaik: +/- together are 2 SD, so the interval [mean - "-";mean + "+"] is 2 SD wide, so the "-" is an estimate of the downwards uncertainly (downwards SD), while the "+" for the upwards uncertainty. In that case, you still have to multiply by 3 instead of 1.5.
I'm not sure of that though, but checking some SD's from the RT ladder (top 5 currently ranked) gave similar results, so I think my claim is correct:
Then again, by taking -1.5 you'd be taking some sort of "average" of a BayesElo and TS-like correction, which makes sense as well.
A full TS-like penalisation is not really appropriate here as new players could not retrospectively join earlier seasons and would maybe favor older players like me a little too much.
As a separate question: how did you gather the data?