I tried to use their online designer thing and it just kind of went blank screen on me, so I eventually gave up.

It can be as simple as Bruce mentioned above or as complicated as calculating sponsor equity (if you're syndicating), account for some years of interest only, calculate seller financed 2nd mortgage, etc.I recommend spending about $100 bucks and getting Michael Blank's Syndicated Deal Analyzer spreadsheet.

We use Michael Blank's SDA and I've seen a lot of people using that model as well.

Now from the outside looking in and not knowing much about you since your profile is blank, i would conclude you are making a huge mistake pursuing a SFH over a duplex in Austin TX especially if you plan on vacating in 2 years and will be renting out said property.  

.* Do you have plans to go ahead and finish office and restroom out for each unit, or are you going to leave it blank and build   the inside to suit? 

Central Florida has great potential to invest in .. i presently work with a couple out of state investors ..each on seperate projects but never during the same time .. from time to time i come across deals like one i am staring in a week or so ..

Here is a link to a blank one. http://www.easternctrealtors.com/assets/files/ecarTraining/Purchase%20&%20Sales%20Revised%2091808.pdf

It is awesome that you are staring this early.

You need an agent willing to write a lot of offers and you may have to look at a lot of homes but this is cheaper than staring a direct mail campaign.

Then after that everything went blank.