Newb help with “making the numbers work”

[Wesley Myers]

Fairly new to real estate we currently have one STR in PCB that's doing well in its second year of ownership. We own it 100%. We have enough "extra" funds for either 2 more places around $300k each putting 25% down or either one more expensive place with 25% down. This all while having a small reserve for each or just the one.

“Making the numbers work” gets confusing to me at times. Help me understand in easy terms what calculations I should be doing to figure out what returns I can expect from a potential property. For percentages what is considered “doable” and what would be killing it? Should I be making these calculations on the amount I’ve initially invested (down payment) or the total price for unit? Have never used AirDNA should I be? 

Any help you can provide would be amazing. Thanks!