On page 116 of The British Industrial Revolution in Global Perspective Robert Allen writes
"...the total factor productivity of enclosed farms was 18 per cent higher than that of open fields. If rent accounted for one-third of revenues, then enclosure boosted rent by 64 per cent, for example a rise from 12 shillings to 20 shillings per acre."
Is the 64% rent increase derived from the 18% total factor productivity increase here? If so, what is the working in detail?