Oldbayrunner:
Another consideration is St Andy's is a par 72 whereas Bethpage Black is a Par 70, therefore it takes a lower score on a par 72 to get lower than your average than on a par 70.
I'm sorry to say that this is not true. For the average only the gross scores count, a 70 is a 70 and a 72 is a 72. 9 hole scores are doubled, 33 => 66.
P: Your average is well saturated (>25) as you have at least 200 combined TP stroke play and CTTH rounds in the score history.
Therefore you don't need to shoot below your average (though this certainly helps), but lower than the highest score in the calculation. This might be 69 for example, thus only lower scores change anything.
That said, the 69 on STA doesn't move anything. Alas, -6 on Bethpage (full round) is 64, well below the average (sic!) for a change of -0.2.
This result caused a change of the score sum by 0.2*25 = 5, thus eliminating a 69 (64+5) in the mix.
Now you need to shave off another 4 strokes from the calculation to become a Master.
Have fun!