My question to Mike is purely mathematical: how much market must loose in order for me to start losing money from my paycheck?

Mathematically, that would be the correct way of doing it.

I see now that even a mathematics whiz wouldn't be able to figure this out....it ultimately depends on what gets built or how we decide to invest proceeds. 

It will not mathematically be the best properties but will offer a bit more protection in terms of having contingencies and inspections etc.

I would use the savings because otherwise you are borrowing money at 6% so you can keep your cash in savings earning 1.75% which doesn't work out too well mathematically speaking

I have a bachelor's in mathematics.

However, because the partner-contractor is responsible for that 10k worth of labor, now he mathematically has his 30% skin in the game, plus 10k. 

The mathematics of emotion are what really puts money in our pockets, and that's what makes real estate a good business to be in.

Modern, direct vented, natural gas and propane furnaces are typically in the range of 92 - 95% efficient.Your decision on whether to replace the furnace will depend on the efficiency of the current unit and whether it make more business sense to replace the furnace or to add a heat-pump with an exchanger insert to the existing system ... and, it is (or should be) primarily a mathematical decision yielding which ever approach is going to offer the shorter payback and lower operating costs.The math aside, there will be other non-math factors.  

and as one ages like a fine wine you figure out that equity is your friend and limit your debt when you can … granted mathematically everyone should do 100% financing and never have an ounce of equity tied up.. but that's just mathematically.. in a down turn like what seems to be running through everyones minds these days because there are a few headline grabbing articles that scare people.. we then start to talk about what to do and how to be positioned...