If someone else launched a company "Fogdeam", which sold similar software to yours, surely you'd go after them.
I don't know for sure that we would. But I'd argue that, being a longer word, you get more confusion with the one character change, since any one character changes a smaller percentage of the overall word.
The position of the character definitely matters to. For example, in your "fogdeam" case, I'd say it's iffy. But if somebody launched Dogbeam, I doubt the thought of going after them would even enter my mind. This is all, of course, assuming that they didn't also use a similar logo and similar messaging, etc.