you can't base spellings purely on whether they look right

You could back in the good old days when people generally spelled things right. Learning the body of English words by rote is nigh unto impossible but if you're used to seeing them spelled right then if they're spelled wrong they look funny. Sometimes I'll try a few versions until I get one that doesn't look funny. Some have been so badly abused lately that it's hard to tell sometimes. I've gotten so that its and it's both look wrong in all contexts.

Well, maybe not purely.