I wouldn't be doing this :suspiciouif your fan is rated at 1.2 amps you need a 10 OHM / 15 watt resister across the red/black wire; with the white wire (signal) going to the negative of the header. @ .42 amps you will need a 5 watt 28 OHM resister.
say you replace the fan with another type. (ie. not the original) , the fan works, but it will still give the error.. why? resistance. the original fan i had was 1.2 amp 12v blower fan. Yes, i said 1.2 amps SOME of them have 0.42 amps (how the motherboard supported this i do not know, i will see if i can get a model number of that fan) this is a 4600C Dell, which I love BTW except that i cant get any fan speeds or anything under lm_sensors for some reason, looking for help on this myself)
if your fan is rated at 1.2 amps you need a 10 OHM / 15 watt resister across the red/black wire; with the white wire (signal) going to the negative of the header. @ .42 amps you will need a 5 watt 28 OHM resister.
Regardless of what is "proper" and the fact that you can "buy the correct part" ... I find your solution to be interesting, experimental, and ultimately impressive. Sometimes there are no spare parts available and it takes a custom modification to get the job done. Your math is not exact, but it is close enough to not fry the board, and therefore ... if it works then it is a solution!