The NFPA Code is Performance Based, that is the fire alarm system to perform to a standard laid out in the Code. To that end, the requirement is that the Fire Detection and Alarm System (FDAS) is going to still detect fires and sound the alarm for 5 minutes at the end of a 24 hour power blackout.
On the other hand, designing the power supply voltages (the Functional Design) is a decision left up to the manufacturers.
Each manufacture's model of equipment is slightly different. The power supply could be a 12 volt nominal system, where the voltage normally runs between 10.2 volts and 13.7 volts. Then again, the power supply could be a 24 volt nominal system, where the voltage normally runs between 20.4 volts and 27.4 volts. From the manufactures point of view, it doesn't matter what voltage is used for the functional design, so long as the equipment and system performs according to the NFPA Code.
When deciding what voltage to use for the Notification Appliance Circuit (NAC) voltage drop, the voltage used has to be in compliance with the manufacture's voltage specifications for the particular make and model of fire alarm system.
If the manufacturer has designed a power supply that produces voltages anywhere from 20.4 volts to 27.4 volts, then the NAC circuit, along with all the horns and strobes have to perform with any of those voltages, or the circuit is out of compliance with the manufacture's specifications. If the NAC can provide enough power to keep the strobes flashing at 24 volts but not at 20.4 volts, then the NAC circuit is out of compliance.
To find out the exact voltages that need to be complied with, you have to talk to the manufacture's technical support. It may be that the maintenance manual or installation manual only shows "nominal" voltages. If so, to find out what "nominal" means, talk to technical support.
Remember, the NFPA Code only shows how a system is supposed to perform during a power blackout; exact voltages aren't in the performance specifications. The manufacture's specifications show the exact voltage to use for the calculations. When calculating the NAC voltage drop, it's the manufacturer's specifications that have to be complied with.
As far as the "Code Compliance" in the System Sensor White Paper goes, the first suggestion is that you contact System Sensor to get the information. However, that is probably not going to get you very far because the "Code Compliance" is a whole series of indirect codes that eventually say that the Type Acceptance of a manufacturers system has to meet performance standards.
You, when making the NAC voltage drop calculations have to comply with the manufacturer's specifications for the type accepted equipment. To be type accepted, the equipment has to comply with a testing laboratory's specifications. The testing laboratory has to comply with the NFPA Code.
Complicated? Yes. Easy to comply with? No. The rules are there, they're just not found in a single place, there's a whole series of indirect rules being covered by many documents.