That is a gap of $41.67/month extra you could be getting if you went back on the market.If it were me, I would compromise and raise rent to $1,213/month or some random number under $1,300 to look like you did some mathematically formula that makes sense. 

There is no formula. 

I had a realtor tell me that basement sqft has a formula of about 50% actual increase in sqftage but I am not sure if that was just relative to appraisal.  

The reason appreciation is not discussed as much as cash flow is becuase there is no set method/formula for calculating the value of your invesment five, ten, or twenty plus years down the road, this is due to the fact that the property is dependent on market conditions.

You see, contractors have a precise mathematical formula on how they charge, and if you think a contractor’s fee is based on their level of expertise, the scope of the job, cost of materials, and or other factors that would make sense, well then, you’d be totally wrong on any one of those instances.Contractors have their method of pricing, and it’s all about you!

What formulas can I be using on my own and what numbers/stats should I be looking at and analyzing? 

You can use it right away if you get a lease for that third unit (75% of gross - PITIA is the formula).

Do you think using a cost approach formula would work in this scenario by adjusting the comps via increasing the total value of the straight line depreciation by the delta of the age?

@Ockert Kruger the formula doesn't change: NOI/cap rate.  

You can use this and the simple formula j Scott has in his book to get estimate of roof including the pitch.