The maths is easy. Just read it out in words.........to fall 1 in 80 means the roof falls 1mm for every 80mm along. If the fall is across the short dimension, then 1 in 80 of 850 is 1/80 x 850 = 10.6mm If it is along the long dimension, then 1/80 x 1800 = 22.5mm. I would hesitate to recommend 1 in 80, though, because dimples in the worked lead can retain puddles when the roof is as fleet as that. I'd be happier with 1 in 60, which gives falls of 14mm or 30mm for your dimensions.
However, it isn't as simple as that, as W2S has so rightly said. These dimensions require more than one sheet of lead, and lead must be joined at a step (from memory that step has to be 50mm, but do check on that .....I'm busy!) if the join is across a down-slope. If you start getting into Code 6 lead, you're A/ getting very pricey, and B/ in the territory of specialist lead workers.