It's hard to measure goodwill in terms of ROI.  

No one wants to work for free, and one of the metrics used internally to measure us is lock pull through percentage. 

Measure three times-cut once.

I also use the 1% rule as a baseline measure of cashflow, if the monthly rents are 1% of purchase price, then the property will cashflow (not withstanding major capex or turnover).

We aim for quarterly maintenance inspections, focusing on different tasks according to the season.

I prefer measurable goals that can be broken down by month and week so I can continue assess where I am and where im going through the year. 

I have made several posts like that over the years, asking what people do to stress test their portfolio, evaluate vacancy rates, reserves, etc.The majority of this board are people who have been in real estate investing for 1-10 years (myself included) and think they're brilliant because they wholesaled some house, did a flip, in an era of incredibly easy money.What I have been doing for a few years is measuring how much rent would have to drop for my properties to break even. 10%, 20%, 30% all the way to 40-50%.I do not allow the total portfolio to go above 75% LTV because you can't refi in an emergency.I also keep 6 months of capital per property.

That is exactly what I was aiming for here...inspiring others to get out there and take action!  

I've done research on rental properties and negative cash flow but am unclear how to actually measure if something is a good investment.

If would suggest finding a contractor familiar with improving the building envelope (air sealing and insulation) for buildings the vintage of yours as applying techniques used for modern construction on older buildings that were not designed to be bottle tight, may require additional measures to avoid inadvertent - and undesirable - outcomes.