I think I have a good handle on this as I've been thinking about it for a while. Currently, my wife and I have about $700k in investments and $150k remaining on our mortgage.

Because of turmoil at my work, we've been looking at our numbers quite a bit. Should we pay off the mortgage? How much is our barebones FI? Can we crush some of the groceries? Do we really need to do the hardwood floors?

And as such, it turns out to be some simple math. End result is...

**We're there!!!**

Currently, our monthly spending is about $5k. Not very mustachian, but that's in Canada with a family of 4. $2k/month of that is mortgage and kids RESP/College savings. This could stop at any time by paying off the mortgage and reducing our monthly expenses down to $3k/month. $2k a month is for Insurance, food, gas, heat, hydro, property taxes, etc. The remaining $1k of that is extra stuff. For example, this month it was a new iPad and tickets to two music concerts. Next month it might be activities for the kids or a little vacation.

So currently, our expenses living exactly as we are is $5k/month or $60000k / year between us. But that's today with a mortgage and kids. It drops by a lot without those expenses.

Now, what we really need to do is assume we're shutting off the kids RESP's and the mortgage is paid off. So then we use $3k/month as a our retirement number since we're living well and happily and not feeling deprived at all. So assuming we have $700k now and we both have decades before traditional retirement age, it goes like this.

We need $900k to generate $36k a year with a 4% SWR. Assuming a 4% return on our investments we'll get there in about 7 years. Between now and then, we need to earn $5k a month to keep paying down the mortgage and keep up with the kids RESPS. My wife's take home is currently $2500/month. That means that I would need to cover the other $2500. Well, if we drop our income in Canada, we'll get about $1k a month tax free in Child Benefits for the next 7 years (Until oldest turns 18). So then there's just $1500/month that we need to come up with. And because I work in technology, I don't think that earning $1500/month would be terribly difficult and on my own terms working on fun projects, that satisfies the OP's question of "fun jobs."

Currently though, we're adding about $60k a year to our stache through savings. Keeping on doing what we're doing, we'll get there even faster. But it depends on circumstances, etc.

So our plan in that in 10 years when our youngest is, theoretically, in post secondary, the mortgage is paid off, we're no longer contributing to RESPs, we'll have $900k in investments. We'll draw down $3k/month at that point and technically be fully FIRE. Between now and 10 years from now we need to earn enough to live. As shown above, we could do it pretty easily. And that's not taking into account CPP, OAS, any inheritance or things that would only increase our net worth and income.

So to the OP's question of *"how much would we need to be FI if we kept working for fun?"* I'm going to say $700k which is where we are right now. VERY GOOD FEELING!