How to Calc End Voltage of Battery After Time?

I recently read in a DoD specification for a fire alarm mass notification system that it was required that I perform a voltage drop calculation for my notification circuits that utilizes the end voltage of the battery test load and duration as the starting voltage of the voltage drop (v-drop) calc.

Typically, I just use the 20.4 V (85%) as the starting voltage for V-drops.

I honestly have no idea how to calculate the end voltage of a battery bank after a prescribed loading period. I did some digging online and came across Peukert's Law (which could be helpful), and in the battery manufacturer data it provides max load capacities for an array of durations: 5 minutes to 20 hrs, and gives the end voltage per cell of these specific scenarios, but it appears to be a dubious proposition to extrapolate or interpolate values from the tables.

For instance, the table tells me that for 5 minutes a 120 Ah battery can discharge at 313.2 A and have a remaining voltage of 1.7 V/cell (which is 10.2 V/batt and 20.4 V/sys), but it does not tell me what the voltage drop would be for a much smaller load over this duration.

Does anyone know how to use these tables in order to estimate the ending battery voltage, let's say after a period of 48 hours under 1 A, and a successive period of 15 minutes at 18 A, utilizing a set of batteries that are 12v and 120AH? 

The battery tables mentioned above can be found in the product literature for Power Sonic PS-121200 batteries.

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