If I do a calculation like this, I always keep the distance units as part of the math to help avoid confusion.

f= GN/ distance(feet)

f=25 / 12 (feet) = 2 or
f=25 / 6 (feet) = 4

So at 12 feet (flash to subject distance) use f2.0 and at 6 feet use f 4.0.

Also make sure that the guide number for the flash is given in the same distance units that you use to calculate. The flash may have a standard Guide Number stated such as Guide Number=25 meters at ISO 100. If the flash guide number is given in meters, then you need to use meters to do the calculation. If the guide number is given for ISO 100 and you are using 400 film, then you will need to close down two additional stops to compensate for the faster film.