Oh and I have also seen some appraiser use a GRM (Gross Rent Multiplier) so basically it's the annual gross rent multiple.

I don't know what the seasonality for that market is but if you can figure out the average nightly rate for the year then you multiply that number by the amount of weekend days you estimate will be booked.

That will give you a multiplier once you understand what the NOI of the operation is and what you are actually selling as a revenue stream.  

Do I just take the price per square foot, multiply that by a decent sized bedroom and subtract that from the sold price for the ARV Comp?

Worst case then, take your 5 best comps, come up with the average $/sq ft, and multiply that by your sq ft.

FHA will calculate the subject properties as follows (the rent of the total number of units - 1) multiplied by .75. 

So, you go through the process you described for each of them, only to have the client tell you that only 1 of them will move forward, or worse yet, NONE of them do.Now multiply that by 10 clients that call you....each of them for their 25 properties that all need you to quote them out.NOW you are within the realm of what a Realtor deals with in their every day life.Again, I explain this NOT to begin any kind of argument with you, but rather to offer you a better understanding of what Realtors deal with. 

Then, from multiplying this number by 12 months, I get $13,548 total.

.** Purchase & Rehab **Purchase Price: $115,000 ($76/sq. ft.)Purchase Costs: $3,450Rehab Costs: $7,500Down Payment: $28,750Total Cash Needed: $39,700** Financing **Loan Type: AmortizingLoan Amount: $86,250Loan to Value: 75%Loan Term: 15 YearsInterest Rate: 4.5%Monthly Payment: $660** Cash Flow (Monthly) **Rent: $1,260Vacancy: -$126 (10%)Expenses: -$504 (44.4%)NOI: $630Mortgage Payment: -$660Cash Flow: -$30** Returns & Ratios **Cap Rate: 6.6%Cash on Cash: -0.9%Return on Investment: -3.9%Return on Equity: -0.9%Internal Rate of Return: -3.9%Rent to Value: 1.1%Gross Rent Multiplier: 7.6Debt Coverage Ratio: 1

Maybe a price per square foot or some sort of factor I can multiply by the difference in square footage.