$1,800 (10%) = $180
jay pays 10% of the expenses beyond the deductible so you multiply the amount beyond the deductible by 10%.
$1,800 - $180 = $1,620
or
$1,800 (90%) = $1,620
you subtract what jay pays from the total or you multiply the total by 90%
jay pays $180 of the expenses past the deductible (not including his $200 deductible) and his insurance company pays $1,620.