If you have cost overruns the exceed your estimates, you don't have any cushion.

You need more of a cash cushion to get into real estate.

I had a budget cushion, so it was fine.

Flipping and BRRRRing is a lot less stressful when you have some cushion in the bank account.

Also, how big of a cash cushion should I have before getting into a multifamily deal? 

For some that's reasonable, but given this is your first flip I would look for more cushion in case something goes wrong here.

It would either help fund another rental, pay down other higher-interest debts, go towards improvements on my current primary, or serve as an emergency cushion. 

Since the market is overheated and everyone is chasing for yield, it's extremely important to be careful with your underwriting process and don't jump in without enough cushion. 

My goal is to pay the mortgage and expenses with that money, that way, any income from the tenants is extra, I see this as a great way to get a solid 2-3 years of payments in, as well as building up a cushion for problems, and future investments, and If I am in between tenants I am not "loosing money" so much as lacking extra income.I will be aiming for the Austin area or between Round Rock and Austin.

Everyone has different comfort levels and as long as I still have a decent cushion of credit it doesn't bother me.