I approximated 7% for property management, 10% for maintenence, and 0% for CapEx.

I would completely agree with the idea of putting at least some of the funds in an interest bearing account if you were self managing and not a licensed agent. 

I do not have a PM, but actively manage my STR. 

The key for me was having the right teams in place (realtor/lender/property manager) to help me find / fund and stabilize the properties. 

I am looking at out of state rentals right now, just doing research, and I was wondering if it was possible to manage an out of state rental property by yourself instead of hiring a property manager.

I also feel I could manage it myself.

What do you do if your tenant is 2-3 days late on the rent?What do you do if he is one week late? Do you always charge late fees or for a good tenant you will not charge? 

I would equate it to managing yourself when you are not set up to manage properties.There is scalability in this...Now to play the Devils advocate if you get autopayments set-up you can always figure out how much interest was paid each year and what your payoff is so just decide how competent you are in that and be honest with yourself, there is nothing wrong in outsourcing parts of your business.

I hire a property management company to take care of all the management side of our properties. 

I've read in a book that Real Estate Investment in Japan is often referred to as a "Scientific Investment" because it can be managed with profitability that is close to the simulation of earnings based on prior calculations of yields.