To answer your question, everyone's good rental return rate is different.
Most newer investors start out thinking 10% is fine but when you factor in being a landlord and unexpected problems, it quickly becomes apparent, "maybe that wasn't such a good rate of return".
Most long term professional investors, tend to shoot for 15% and above. If 15% is good enough for Warren Buffet, it should be good enough for you.
With that said, returns are in direct correlation to the amount of risk you are taking on. Risk is based on leverage, neighborhood, investment time frames and your personal situation. I can think of a several more but lets keep it simple here.
As an example; a new investor might be able to borrow 70% of the purchase price and so long as they maintain a real job in case they lose a tenant and still be able to afford the mortgage; risk is mitigated. If they lose their job, risk goes up significantly.
I purchase properties for cash, so my risk becomes tying up capital in a under-performing property, maybe only getting 10%.
So again, everyone desired rate of return is different. Baltimore offers returns from less than zero to some serious rates of returns.
Because I've been an investor for 30 years, I shoot for returns of 20% plus. Sometimes finding deals that can generate returns of 25-30% per year. But one has to have a lot of patience and experience and access to capital to pull that off.
It would be wise to work with a real estate agent that actually invest in real estate and has experience. And discuss your desires and current situation so they can help you determine what might or might not be best for your particular situation. There are some good agents around Baltimore.
Hope that helps.