I guess the question is, could someone predict the shrink/ growth of that MACD black bar, right? Is this so, first poster? I think u just use the same principle, like what we all use in anything. Follow the trend. If there is a narrowing/ lowering of the black bar, then there is a higher chance of it to keep narrowing. And conversely as well, until u see a sign of reversal of the trend. If you take just the black bar and see it, u can see trendlines, resistance/ support, channels, and more.....including comparisons of current peak with previous wave's peak...etc.. Just....Look at the black bar.
In fact if you can program it, I think u can take this black bar and then run a signal/ indicator on it. But I'm not sure this is necessary, just naked eye will do.