A more comprehensive STR marker derived TMRCA estimate for the two tested samples in this branch is a few hundred years more recent than TMRCA via SNP dating method from Yfull.

I just completed some additional research looking at all the STR results for Guess (86068, YF06431), and Mitchell (249822, YF01489, YF06577) from their two 111 marker tests, one BigY test (Mitchell) and two FGC Y Elite tests.

On YFull's STR matching data tool there was 263 STRs compared between Mitchell's FGC test and Guess' FGC test of which 23 were different. Mitchell's BigY test and Guess' FGC test had 259 STRs compared, of which 23 were different (not the exact same 23 markers).

I cut and pasted this YFull data into a spreadsheet, and used the FTDNA 111 marker test results to fill in many blanks in the BigY and FGC test data.

This resulted in ~80 additional STRs (339 markers total) to compare between the two samples, with 29 being different.

I entered this data into the McDonald's TMRCA tool at

http://clandonaldusa.org/index.php/tmrca-calculator
And got the following results using 0.0026 mutation rate.

Generations Probability for TMRCA

15 41.2%

16 54.7%

17 67.2%

18 77.7%

19 85.7%

20 91.3%

21 95.0%

22 97.3%

Using 30 years per generation there's a ~50% probability TMRCA lived no later than 1500 AD, and ~97% chance he lived no later than 1300 AD.

As stated earlier, this is more recent than the SNP dating method for TMRCA used by Yfull which is currently ~1000 AD (95% CI 450 AD to 1350 AD).

There is a small overlap between the STR and SNP dating methods (1300 to 1350 AD), so my best guess at the moment is TMRCA for this branch is 1325 AD.

The addition of the new BigY test sample (see post above this one) and his future 111 marker test will further refine this estimate.