I'm a huge fan of this question and my thinking has changed over the years with my circumstances. Immediately post college I built up a decent emergency fund, the full 6 month of necessary expenses. (hey my dad told me to max out my 401k and do the 6 months fund so I did) After a few years I realized by showing up on time, doing decent work and basically having less BS than my coworkers, I was never going to get fired/laid off.
At that point I kept a month or so in the checking account and invested everything else.This was in my 20s when I was not only a low cost operation, but very low responsibilities.
Somewhere in this time, I got an engineering license and was pretty confident that if I got laid off, there were a bunch of firms that would pick me up really quickly if that's what I wanted. Maybe not my ideal pay rate or job, but I could be cash flow positive and the resume would still show my chosen profession. If your local labor situation isn't as friendly then this maybe doesn't apply. This has also made me relatively risk tolerant as I got older and took jobs with start ups ect where I did get laid off/ they went belly up.
We also avoid debt and therefore pre-save for big expenses. So that means we've had money for travel and hobby stuff accumulating in a savings account as well as major expenses. I historically did NOT count this towards an emergency fund as employment breaks for me are travel time so that money was going to get used.
But now that I'm older, the savings for car expenses, this and that house project or furniture, the kids summer camp ect has gotten quite a bit more extensive. If I'm unemployed this summer, I'm selling the kids tablets and dragging the kids off into the forest for a long national forrest camping trip. Then we can spend the summer program money covering our living expenses. We have so many more categories of savings now that it seems silly not to view this money as useable.
I also became an independent contractor in the last few years and want to stay that way. Because of that I keep a much larger, ~4 mo reserve on hand for layoff time. So far I've been able to keep myself booked up with work but contracts fall through and get delayed all the time. I could still go back to being an employee if the layoff time lasted too long, but I want to make this independent thing work so that requires more $$ on hand to ride out lean times.
Its also psychological. The HELOC thing got mentioned and I'm totally OK with it. Part of the MMM style path is you have a lot of freedom to experiment with what is optimal and if you are wrong, its not a big deal, you pay to much in interest for a bit and try to do it better next time. I suspect everyone on this board is doing more than enough things right to cover the odd mistake. However, the Missus Troll abhors debt more than I do and wouldn't be happy if we drew on the HELOC instead of selling investments. She wouldn't be happy about selling investments either , so we keep more cash on hand.
Housing Costs got mentioned, and this is a topic we haven't sorted out yet. I would be ok using the HELOC or refinancing for something like a new roof. Borrowing for a capital improvement to a capital asset is something I am ok with.