Tweaked the numbers in the spreadsheet to minimize the cumulative relative error.
There was a large relative error in the higher baud rates, now the relative error is minimized, while keeping the functions linear
(not extensively tested yet)
// 16MHZ
rxstop = 16000000L/(7 * baudrate) - 2;
rxintra = rxstop;
tx = rxstop - 4;
rxcenter = rxstop/2 - 7;
// 8MHZ
rxstop = 8000000L/(7 * baudrate) - 4;
rxintra = rxstop;
tx = rxstop - 2;
rxcenter = rxstop/2 - 10;
// 20MHZ
rxstop = 20000000L/(7 * baudrate) - 3;
rxintra = rxstop;
tx = rxstop - 3;
rxcenter = rxstop/2 - 7;
to be continued...