Trailer tie down straps

[mk4gtiturbo]

Is there a magic formula to work out what straps to use on my trailer?

The car is 1250kg and I have 2x 1000daN (2000 breaking) straps left over from the Caterham. I was thinking of these two at the back plus 2x 2000kg at the front.

What is the general rule of thumb?