Powers must be in linear units (i.e. watts), doesen't matter if you use multiple or submultiples but Pr and Pf must be espressed in the same units. f.i
Pr in W and Pf in W
Pr in uW and Pf in uW
and so on.
However your case is not a real one, since the reflected power is higher than the forward power. If, instead we have Pf(dB)=1 dBm and Pr(dB)=-0.5 dBm we will have:
Pf = 10^0.1 = 1.26 mW
Pr = 10^-0.05 = 0.9 mW
then SWR = [1+sqrt(0.9/1.26)]/[1-sqrt(0.9/1.26)] =11.9