"My back has to be vertical..."?
"Every coach agrees with this method..."?
Either you're just trolling or you haven't read Starting Strength at all. The picture you pointed to shows the toes jammed up against the wall and a woman squatting *above parallel* (it may well be impossible to squat correctly jammed against the wall that way).
Go back to the book and either film yourself squatting or get a friend to spot you who has also read the book.
It's not necessarily easy to find correct form, but you'll have to at least have an empty bar on your back before you can judge.