HI Jesse,
First, Thank you for your service! I wish best of luck finding a place that will work for you.
That being said, I hate the 50% rule! As you already mentioned, it is a guideline. Using 50% or 60% on this property will throw the numbers way out of whack.
Do you know what the taxes and ins. will be? Does your debt service number include these as part of a PITI payment? Or are you separating them out? The 50% rule is including taxes and ins.
The biggest factors are going to be:
Management (10%), which you will likely manage on your own to start, then need to contact out when you need to move on.
Vacancy (5%-10%) depending on area. Even if you are renting at 100% all the time, I would never go below a 5% vacancy factor.
Maintenance (10%) probably lower for new construction, but I would budget this in and build up a reserve.
The other expenses are going to be the water, sewer and trash which you should be able to get from the town/county as well as the seller.
If you have the Tax and insurance #'s you should be able to come up with a more accurate number based on the actual expenses instead of the 50% guideline.
All the Best!