Annual income:54*600*10=324000
Cost:
tax: 30k
util:200*18*12=43200
insurance:5k
maintenance:20k
Net: 225800.
You have not included furnishing of 18 3br units. 7kx18=126k (furniture in each bedroom and common area,window treatment,appliances)
So the number is 225800/2426000.
Actually I think your maintenance will be way higher than 20k(cleaning, ground upkeeping, parking), because this is a 1800/m apartment, nice property, not a poor house. You have to spend money to keep it nice.