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So you originally have 2 flats each with their own supply and meter and then the upper flat was split into two flats with the electricity supply from the original upper floor flat supplying one flat which has a meter and the supply to the new flat is sub metered from the upper floor flats supply

I'm not sure why you need to calculate meter readings as they are what they are as the meter shows the only calculation needed would be to apportion the standing charge although issues may get complicated if an E7 tariff is involved

Without knowing the current layout and supply arrangement it is difficult to give a definitive answer but your information would suggest you only need one new supply for the sub metered flat which will be billed by whichever supply provider you choose to purchase electricity from