Besides profitability, you cannot ignore liquidity and solvency. They are equally important.
to calculate profitability, you have to take all parameters into considerating, which include, but not limited to:
Property appreciation rate (or depreciation rate which could be more reasonable if treated like a consumable commodity)
Inflation rate
Mortgage interest
Rent yearly grow rate
Alternative investment return rate
......
Based on the current economic situation, 6% is ok provided that both liquidity and solvency are good. Otherwise, it's a headache because you cannot squeeze profitability anymore