Purchase price: $185,000 Cash invested: $35,000 Sale price: $671,000This was our 3rd home.

@David YoungRecommend you first figure out the property Class you want to invest in, THEN figure out the corresponding location to invest in.Property Class will typically dictate the Class of tenant you get, which greatly IMPACTS rental income stability and property maintenance/damage by tenants.If you apply Class A assumptions to a Class B or C purchase, your expectations won’t be met and it may be a financial disaster.If you buy/renovate a property in Class D area to Class A standards, what quality of tenant will you get?

I frequently hear that someone purchased a new property because their real estate agent told them they could get X amount for rent, when in fact, the market comps don't support that amount.

We've never used percentage-based criteria, like the 70% Rule, for purchases.

I currrntly own a home and am renting it out, I am working on purchasing a duplex.

The only two ways to rectify this are (1) purchase the property far below market price or (2) significantly increase the property net income usually through some method of “repositioning”. 

Once I was deployed on a submarine in the North Atlantic and they helped my wife with a 1031 exchange on the sale of a home and purchase of 2 other properties.

I already have an investment analysis workbook built that tells me everything I need to know to make a purchase decision, NOI, COC, ROI, and if the property will achieve the appropriate coverage ratio to be funded. 

Its usually called a post-completion or post-purchase mortgage.

That ensures you don't spend it on other things and know exactly how much you have to spend on the next purchase.