[pcabc]

I'd have a look at some individual charts to confirm the data, as from a quick visual check of 2020 for example, I'm seeing more stocks above their Mansfield RS zero line than not when looking at the Nasdaq 100 stocks for example. This is just a quick visual check mind. But the Percentage above their Mansfield RS zero line in your chart seems far too low.

The simple answer here is to run the calculations on the Nasdaq 100.  This will take some time, but not as much as the Russell 3000 or UK pools.  If the Nasdaq 100 plots correlate with a reasonable sample of NASDAQ 100 stocks then the math are likely good.

OK, there has been a bit of a delay in doing this.  I've produced some new code to tabulate the relevant parameters for each stock in a group of stocks I'm measuring breadth for.  I ran this on the NASDAQ100.  It appears that the correct values on the correct dates are being recorded from doing some spot checks on the individual stock charts.  The overall total seems to match the recorded values saved in my database - with a couple of allowances for incomplete data which makes a small difference.  So, on balance, it looks like the calculations of breadth based on Mansfield Relative Strength are correct.

However, a lot of my stock data starts around 2020 or perhaps a year before.  So, even if the calculations are correct, the 2020 data is not good.  So I'd only trust my Mansfield RS breadth calcs from 2021 onwards.