looks right as far as i can tell, going to be kind of confusing typed out like this compared to written on paper, but anyways:

V = Volts, I = current in Amps, R = resistance in Ohms, P = power in Watts

V = IR -> R = V/I

so for your fan, R = 12V/0.15A = 80 Ohms

resistance for series circuit = R1 + R2

so to get down to 7V, you want to make the fan's resistance equivalent to 7/12 of the total resistance of the circuit

80 * (12/7) = 137 Ohm total -> resistor needs to be 137-80 = 57 Ohms

80 + 57 = 137

so total circuit: 12V = I * 137

I = 0.0876A

or you can calculate series current like this:

1/I (for overall circuit) = 1/I1 + 1/I2

1/I = 1/0.15 + 1/0.21 (since the resistor would be drawing 12V/57 Ohms = 0.21A current if it were by itself)

1/I = 11.4, I = 0.0877A

this is just so we can calculate the power dissipated by the resistor, if you have a big fat 10W resistor or something (or if this makes absolutely no sense) you can ignore everything below "57 Ohms"

P = I^2R

your resistor needs to be able to dissipate 0.0877^2 * 57 = 0.438W of power, so look for one that can handle say 0.5W (and 50 or 60 Ohms is probably about the closest you'll easily find).

buying one resistor at a time usually gets pretty expensive, if you can just buy a mixed bag of them from say 20-200 Ohms, rated for a watt or so, you can mix and match them to get whatever voltage you want for any fan.

edit: as you can see i assumed that "max" current = current draw at 12V; i'm not sure whether manufacturers base maximum on the rated voltage or the point at which the fan explodes.

too lazy to check other fans and see what seems reasonable... if that max current is actually current at 13.8V then you'd need a resistor something more like 66 Ohms to end up really close to 7V.