(b)in the case of an insurer carrying on solely re-insurance business one-fourth of one per cent of the total premiums in respect of facultative re-insurances accepted by the insurer in that class of insurance business during the year preceding the year in which the application for renewal of registration is required to be made (the amount of fees so arrived at, if not an integral number of rupees, being rounded off to the next lower integral rupee), or five hundred rupees, whichever is greater].