Is that 86.48% value from the regression calculation, or from tuning, or...?

It's the outcome of the

mean of the [NYSE %advn] PLUS 2x standard deviation of [NYSE %advn]

where the 2 was chosen to approximate the 90% (as 2.5x - 3x - 95.6%-104% - would pretty much 'never' fire due to the normal distribution being only an approximation of the actual distribution) where the mean (49.97%) & sd (18.26%) are both calculated since 2005-10-27 (the complete data series I retain). So *somewhat* tuned - but no real effort to optimize it - and in practice 85%-87% works better than 90% when combined with the 'improved volume'. 90% is just too restrictive when combined with the 2nd criteria, but works a better than 86.48% when used it's own single criteria. I expect that a one could find a better figure - the 'mound of toast' range appears quite broad, but I've made no real effort to explore it widely preferring to obtain something that works for the data I have & doesn't seem too tightly tuned.

(Interestingly using 85%-86% provides a re-entry point for 2011 on 2011-10-27 and so offers further improved performance across the data set at the potential expense of further curve fitting - YMMV. It's tricky however 85% or 86% may well be what folks prefer to roll with. Part of the reason with posting all this is to let other folks express an opinion on the tuning or otherwise.)

One of the potential 'builds' I'm considering is just to measure the mean & standard deviation across only a year or two to allow for it to flex a bit, but I've not really explored that yet.

Platy

(I note that despite extensive checking of the first post I wrote "… where the 86.48% is the mean - 2 standard deviations of the [NYSE %advn]" this should be "… where the 86.48% is the mean + 2 standard deviations of the [NYSE %advn]". Apologies.)