BiggerPockets has a STR calculator that can help you figure out expenses, long-term cash flow, and more you can check out as well when you get your numbers going.

Listen to the Bigger Pockets Real Estate rookie podcast... you wont understand everything at first, but soon it becomes like your own language.Meet people in your area... network.. tell them what you are interested in, you will be so surprised that people will show up to help you.Avoid the gurus who ask for money, PRO membership here gives you access to the webinars and calculators you need to make it make sense for a newbie. 

Calculating the cash flow for both properties:Property A: $1,800 x 12 - $2,000 - $1,500 - $250 x 12 = $15,100/YrProperty B: $1,600 x 12 - $800 - $600 - $35 x 12 = $17,380Property B is the better property once operating costs are included.SummaryBecause the rent price ratio does not include recurring costs, it almost always provides invalid results.

Sounds like you calculate that the loss of 4-6 mo of rent of the Spring semester is adequately offset by the premium rent you expect to receive starting in June.

For market pricing, I collect LTR data from BP rent calculator and multiply by 1.5x.

How should I calculate loss of use for 3 weeks?

Does anyone have a recommendation for online calculators to map out the breakdown of monthly interest payouts easy?

Bookkeeping when it comes to real estate investments can get tricky.I often find the following mistakes occur1) They fully write off the mortgage payment when in reality the mortgage payment is broken down into Interest, principal paydown and an increase into the escrow account.2) If the property is managed by a PM company, do not factor in the PM Statement3) do not calculate depreciation expenseBest of luck in your search for a proper bookkeper.

CoCR is only calculated in the first year of ownership. 

Here's a bit more in detail about how rates are calculated for DSCR loans:1.