In addition, a percentage of the rent of the 2nd unit would count against the mortgage payment providing some assistance at qualifying for the loan. 

I didn't mention it because I figured that was a given.However, your example of it really isn't the right application of it, but it's a common one since most people try to use Stocks Market mentality and analysis for REI...and they have very little in common.You should never compare percentages in REI. 

IF you had to replace a roof, HVAC, driveway/parking, kitchen and/or bath, etc...after that time period (5 years), or worse, during that time period, and your retentions didn't cover it (most likely), it would mean you didn't break even on your cost.I despise (because I can't think of a stronger word) using percentages as results or factors in analysis. 

In some ways it will be easier to show a profit since you are living there so you will have to come up with a percentage split to be applied to the expenses.So, it helps that you don't have as much deductions applied --- you don't have to artificially inflate the rental income. 

If I were you, I'd also consider lending money to investors at a higher percentage rate.

Since the rate and assessed value will probably not be on the same page you will need find them individually and then multiply the rate(Convert into percentage if displayed in another form) by the assessed value to come up with the property tax of the home. 

Certainly, we can all agree that every segment of society has a few bad actors.However, in your post, you set out haranguing other landlords (that posted above, as you state) for some perceived wide-ranging ill that there does not appear to be any evidence for, at least on this thread.To baselessly condemn the entire profession without that merit, to me, does indeed reinforce a false narrative.If I were to muse publicly, as you did, that since a few individuals belonging to an industry (or ethnicity) did wrong, paint them all with a broad unilateral brush and declare “and people wonder why (that group) have a bad name”, I’ll bet you’d use a harsher term than “false narrative” to describe that sentiment.Landlords have a very unfair negative public image because of a very small percentage of individuals in the industry, and you are not helping.

When you come to the part where you are inputting Rental Income there's an option box right there to add a percentage. 

Because my percentage was a lot smaller than my mom's I only own about 5% of this new property.

The FHA is a good way to get started or possibly asking your mortgage lender about a lower percentage down conventional mortgage with PMI, that way the insurance will drop off after 20% of the loan is paid.