One thing I remember from working on temperature co-efficients before is the importance of paying attention to whether the co-efficient is given as a percentage or in volts/degree. If it's in voltage/degree, which I've seen on some spec sheets, than you've got one less step to do.
Good point! Usually it is given in percent per degrees C and it can confuse some people if all of the sudden the coefficient is given in volts or mV per degrees C.
You can also convert mV per degrees C to % per degrees C and vice versa.
Typical 60 cell module: 38Voc, Temp coef = -0.3%/C
Math: -0.3%/C = -0.003/C
More Math: -0.003/C x 38V = -0.114V/C
And to go back
Math: -0.114V/C divided by 38V = -0.003/C
-0.003/C x 100% = -0.3%/C