You are better off getting the value based on the current rents and determining your ARV based on the future rents using the Gross Rent Multiplier.

When the numbers work like they should, you will make more than double the profit with a 20-unit because when you raise the rents every year by even a small amount like $30 per month per unit you make a ton of money when you do the math with the Gross Multiplier.Suppose, your 20-unit property is in an area and in condition where the Gross Multiplier is 18.

Then multiply the property's square footage by the rating.

Leverage is a proven way to multiply your rental income and appreciation value.

The only issue with that calculator @Alex S. is the results tab only calculates January rents and multiplies that by 12.

Look at what your starting pay is and multiply it by ten.

So in this case the tax rate is 2.9348 and it is multiplied by each $100 of the assessed value results in the property tax amount. 

Our calculations show that the 6 unit property will net is a minimum profit of $980,000 in 10 years and if everything goes the wat we project our profit will be $1.2 million, or $120,000 per year for 10 years.So, your best route is to look into selling your homes and purchase no less than a 4-unit property because when you own properties with 4+ units the value of the property is determined by annual income multiplier times the Gross Multiplier.The following image is an example of the profits for a 4-unit property when you increase the rents only $50 every year. 

Learn to do the math and do the calculations that show how much profit you can earn over 5, 10, 20 and 30 years when you raise rents and the rent increases increase the value of the property due to the Gross Multiplier.

Multiply the occupancy % by the rates for the summer months based on what you can find on airbnb and vrbo.