If you are going to make the commitment to marry and take the risk you should be prepared to divide 50/50 if the marriage lasts longer than 5 years.

Then divide that by purchase price to get the ROI or CAP rate. 

What's left (heirs' equity) gets divided up "per stirps" meaning you get what's coming to you, net of the costs. 

You could divide by the numbers of units.

Your rate is primarily determined by how high your debt-service coverage ratio is (basically measured as net income from the property divided by the loan payment).

The other advantage to that is, you may be able to divide the lot and later sell each duplex separately, if you sell.

You can create another income stream by purchasing Mutual Funds or ETF's that provide monthly income via dividend, buy dividing growing stocks (and reinvesting the dividends), or just buy stocks that are trading at a discount.

I divide the gas and electricity among the 4 tenants based on sqare footage. 

I am wondering how most people figure out what number to use, to divide by 27.5?  

I have in the lease that the water bill will be divided by to 2 units, but one of the tenant has a washer and the other one does not.