This is what you will find in the cisco doc

Policing Traffic with CAR

CAR embodies a rate-limiting feature for policing traffic. When policing traffic with CAR, Cisco recommends the following values for the normal and extended burst parameters:

normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds

extended burst = 2 * normal burst

Now why they choose these numbers is unclear. These seem kinda big to me but they may be choosing a value that is safe for most conditions.

The only bad thing about making this too high is that you waste memory in your router. Larger values also tend to make your utilization of your underlining circuit more bursty but it depends on your situation if this will cause any issues.

In general you will need more buffers (ie larger burst) when the ratelimit you are tryng to obtain is a lot lower than than the physical speed of the interface.

It is farily easy to see when you set it too low because the show rate command will show packet drop but the transfer rate will be below the limit you configured.

Thanks pravin

i eventually used this formula: N burst = (configured rate * 1.5)/8, as used in cisco one of cisco's examples on the subject.

Thanks man. that's an intelligent response. i got that info earlier from cisco's site but was very sceptical applying it!