Then, multiply and divide by the 70% and subtract how much the repairs will cost? 

Now you need to look at the benifit of filling the pads...Lot rent is $150 per month... so some quick math...150 (per month) x 12 (months) = your gross income- $1,8001,800 (gross) x .7 (rough multiplier to figure net) = NOI / PAD @ $150 / month - $1,2601,260 (NOI) / (divided by) .11 (CAP RATE) = $11,454 Which is the value of each pad if priced at a 11 CAP-please do not try to tell me your in a 7 or 8 CAP market...

I am interested in getting an approximate value multiplier.

If you're looking for the best multiplier of the funds you have in the paid-for house, purchasing two or three other units would make the most sense.

For a high super-conservative estimate go square footage times ceiling height divided by 27(to convert cubic feet to cubic yards) thats the max amount of dumpster you  could need, then try to estimate the amount of many hours it will take multiplied by the labor rate you are comfortable with. 

One banker told me they would take my gross rents, multiply by 75%, subtract the mortgage, taxes, and insurance (PITI) and count that as income.  

If you are familiar with the market rents then it would be easy and more accurate to utilize Gross Rent Multipliers. 

So, I'm full-time now.Now, it's time to learn about trading money for time like "rich" people do instead of trading time for money like W2's do ... making money work for you like "rich" people do, instead of you working for money like W2's do ... and multiplying yourself by hiring others like "rich" people do.

When determining the value of a 3-4 unit property, which is more useful- the Gross Rent Multiplier or the Capitalization Rate?

Multiply that by your SQFT.