5. Where any holding in respect of which a claim has been made is situated partly outside England, the individual reference quantity of milk which shall be treated as available to a claimant in relation to his land in England shall be calculated as follows—
IRQ = TIRQ × X ÷ Y
where—
“X” is the claimed forage area in hectares of that part of the holding which is situated in England;
“Y” is the total claimed forage area in hectares of that holding;
“TIRQ” is the total individual reference quantity of milk available to the claimant in respect of that holding; and
“IRQ” is the individual reference quantity of milk which is treated as available in respect of that part of the holding which is situated in England.