I think the answer of .78 is correct if the question is restricted to autosomal DNA. It gets more complicated for the mtDNA, yDNA and X-Chromosome. The answer for these other 'types' of DNA are dependent on the specific sex of the children along the seven generations.
yDNA: 100% If it is seven generations along a paternal line. 0% otherwise.
mtDNA: 100% If it is seven generations along a maternal line. 0% otherwise.
X-Chromosome: ?% The answer is dependent on the specific sex of the children along the seven generations. I just checked and WikiTree does this calculation correctly. Sometimes the answer is .78 and sometimes it is not.
I realize Autosomal DNA is the bulk of the DNA, so .78 is always close to the correct answer. However, I think these other types of DNA are much more interesting than autosomal DNA, and much more useful for WikiTree.