[CMOTD]

As Woodcote says.

There is no hard and fast rule. Much depends on your hand/eye coordination. The BSCRA website is a mine of information so here is their advice:

"A frequently asked question is what value resistance controller / how many ohms do I need?  (This is a non issue for adjustable electronic controllers -  just twiddle the knob to give you whatever you need.)  

Unfortunately the what value resistance controller you need may not have a single answer - you may need more than one.

• Motor power - higher powered motors take more current, so a lower resistance is needed to slow them down.  Set against this higher powered motors make the car go faster so they need more slowing down for corners.  Mostly the first factor is more important so higher powered motors most often need a lower resistance controller.
  

• Chassis handling - more power can be applied in corners with a better handling chassis. So the motor doesn't need the power reducing as much. So better handling chassis usually need a lower resistance controller.
  

• Tyre / track grip - cars corner faster with  more grip. So the motor doesn't need slowing down as much. So more grip usually need a lower resistance controller.  That's true whatever is providing the extra grip - for example stickier tyres, magnet traction or aerodynamic downforce.
  

• Tight corners - cars need to go slower round tighter corners. So the motor need slowing down more. So tracks with tighter corners usually need a higher resistance controller.
  

• Track power - cars go faster with more track power. So the motor need slowing down more for corners. So tracks with higher power usually need a higher resistance controller.  This is true for tracks with a higher supply voltage. It is usually true of tracks with better wiring because more of the supply voltage actually reaches the car at the far side of the track.
  

• Driver choice - some driver's driving style is suited to a lower resistance controllers than others.
  

So where do you start? I know some people will have different ideas, but as a starting point:
Basic home set 1/32 45 to 75 ohms 
Quicker home set type cars 15 to 45 ohms"

For the price of two resistor controllers you could buy a basic electronic one which will adjust for any car so I would always recommend going down that route if possible.