Where should I position open fire ventilation grills?

I worked in Local Authority Building Control for thirty plus years. I can tell you that the amount of ventilation, (combustion air) will depend on the sizes of the fire place opening and flue.I ume you have applied at your Local authority Building Control for the appropriate approvals.Contact the District Surveyor who covers your area and he/she will be able to advise you of the requirements for the amount of combustion air and position of grills to provide same.