Well how much makes a 'sizeable percentage'. 10%, 20%? 30%?

Looks like close to ~30% last year (after seeing ARR fall by 30%). [1] I think higher in prior years.

He claims to have spent only a handful of hours on BCC last year. So spending less than say 20 hours on a project and having it generate ~30% of revenue, passively, seems like a pretty good deal.

Another poster mentioned focus, but that doesn't really seem to be the case here.

Of course all this is conjecture.

[1] http://www.kalzumeus.com/2014/12/22/kalzumeus-software-year-...

Patrick is not reporting his full income from AppointmentReminder, he excludes the Enterprise deals.

"Focus" is a very valid reason. Even assuming just one hour per week, that's 52 hours he can spend on something else. Plus having BCC around is a mental burden. Even while you're not working actively on it, you're thinking about it.

At $19,000 of net profit per year, I'd be startled if it was anything more than 20 percent. He was pulling in $30,000 a week[1] doing consulting.

[1] http://www.kalzumeus.com/2015/05/01/talking-about-money/