If we subtract 3 generations between Helen and Boris, she would be 13 generations from president Hayes. But if every generation would mean an offfspring of 2, 13 generations would mean 2^13 = 8192. 18 generations from G Bush would mean 262.000.
Hayes is from Pennsylvania and Helen from neighboring Ohio. PA population 1880: 4M, OH: 3M. So from a total of 7 million, one of Helen's 8000 relatives was president Hayes. Many people at the time probably lived in the countryside which Helen and the president probably did not. (And maybe even fewer people going back in time? Children more numerous than 2. But also considering marriage within the tree etc)
Well, I am just speculating wildly here in a field I do not know. I am among experts so I just make my statement and hope it will be corrected:
Helen's family tree, that connects Johnson to US presidents, maybe shows that she was born into "higher levels of society" in the area, likely to marry a maybe wealthy Russian immigrant. (But this is purely improvising on my part. Please help me straighten this out.)
I look forward to connecting myself to this excellent system in the future. I am not sure how it works outside the US, though.