Davenport No.10 was 855ft long. Using HMS Eagle bow profile an overall length of 865ft at the flight deck would just fit.According to my copy of Friedman the waterline length of the 1952 Carrier was 870 feet, but that the overall length was 815 feet.
Brown in Rebuilding the Royal Navy says that the length was 815 feet, but not whether it was the overall length or the length at the waterline. My guess that this is a printing error and that it should be 815 feet at the waterline and 870 feet overall.
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Hence why they felt 870ft at the flight deck was the limit.
And in turn that meant 815ft at waterline.