Glad I could help. Be careful about assuming too much. Take each step and each generation one at a time. The census can only tell you so much. Death certificates, obituaries and marriage records are very important (most of them are not online). Sometimes you need to use several bits of data to put things together and weigh the evidence. Basically you are a detective and you have to build a case to prove what you believe.
For example there were probably several Mary McGills who were born in Pennsylvania around the same time. Finding one in the census doesn't mean she is the right one, it only means she could be.