HRA can be computed on Basic (as it becomes easy to calculate its impact on income tax), whereas conveyance should be kept independent of Basic. Conveyance of INR 9,600/- is exempted from income tax, and this has been raised in the current budget to INR 19,200/-. So the conveyance amount should be kept fixed at 9,600 or 19,200 so that your employees can avail the maximum benefit regarding income tax.
Regards,
A.B.