How do you go about finding out or determining the proper Gross Rent Multiplier to apply to a house that you are thinking about purchasing?

Multiply that by a vacancy rate.

Material for bathroom redo = $1500 for materials * Multiply by 3 and you would arrive at $4500 for the total job.

As far as age, I have been using a multiplier if it is more than a 10-15 year difference.

As a young investor, there are several things on your side, such as low cash output now (like you should have no or a low car payment, reduced student loans (if any), etc), ability to learn in an orderly fashion about RE investing (and not be rushed) and time is on your side, which can help you multiply your initial investment and give you a chance to expand your knowledge through committing to several RE transactions for the next few years.You say you want to get started?

(think of the worst car salesman stereotypes and multiply by 100x and throw in some illegal sales techniques, that's finance) lol. 

My holding period is around 3 months, so I multiply points by 4 and then add that to the interest rate to figure out my effective annualized interest rate.

That being said, it is imperative that you understand the particular areas average operating costs including taxes, insurance, management, maintenance, utilities, etc.For a quick financial rundown on a property, I take the gross rents, subtract 10% for vacancy, multiply that AGI by 60% (40% oper. xpense) which gives me an estimated NOI.

Now I also take into account idiosyncrasies of the house and the neighboorhood, and whether the house is under or over built, but initially it comes from an average sold per square foot number from the area, multiplied by my square footage.

For example, I've heard some people will multiply the square footage by anything from 10-20 depending on basic observations of the condition to get a rough number for rehab.