This all hinges on probabilities, and only you can really be the judge of that. Your statement "Of course the bonus is never guaranteed" is doing a lot of heavy lifting here. How much you factor that bonus income in depends on how likely you think it is that you'll actually not get a bonus in any given year.

You don't want to be in a situation where one down year causes you to lose your house, so you need to structure things so that won't happen. Maybe that means you have a certain amount set aside in liquid accounts so that you can pay a year (or even two) of mortgage if you don't get a bonus.

A relevant question here: how much do you actually want to spend on a house? Are you just trying to spend as much as humanly possible, or do you have a target in mind? Because there's no point in spending a lot of time and energy figuring out your exact limit if you're not going to come close to that. It seems like you could easily put aside a year or two of mortgage on a really nice house and then never have to worry (about this, at least).