Or is there no such thing?
I haven't put together a case study yet, but for reference I'm at $57,314 gross (salary) plus $13,200 rental income. I can "afford" it but it'll make building my safety net take a bit longer.
Right now I have 6% going to a Roth 401k (50% match) and I just max'd my ESP (NYSE: FIS), which I expected to be 15% of net. Well, I got my first post-change check today. Apparently it's 15% of gross, but pulled out of net income, or $712/month. I get a 33% match (either stock or added to my check) after 12 months, so as of January 2016 I'll be getting $234.96/mo back for free (minus tax).
I have ~$6k in my company 401k and ~$24k in NH State Retirement (which I need to pull out and transfer somewhere else, as I no longer work there). I don't have enough of a safety net to be comfortable right now, but since I've discovered MMM and decided to downsize, I expect that to grow quickly enough that I'm not worried about it.
It makes me nervous having ~$8500 annually going into one stock. I believe I can sell it at any time, but I am not sure of the impact on my employer match if I sell (i.e. I don't know if I just have to keep it for 12 months, or what). I can try to find out, though. I'm also not sure I'm going to stay here for over a year, but if I don't I can sell (and if I do, it's 33% guaranteed return on the initial amount, plus/minus what it gains/loses in the meantime).