Accuracy of i and j for arc centre

Can anybody tell me how accurate i and j have to be.
In metric i am rounding down to 3 decimal places and in imperial im
rounding down to 4 decimal places.This could lead to a 0.001 error
in metric and a 0.0001 error in imerial. would this be a problem?

I dont want to use r instead of i and j

thanks,

sid