i believe the ratio is the length of one side of the fulcrum compared to the length of the other side of the fulcrum. If u know your lift divide it by 1.6 and this is your cam lift or if u know you cam lift u just take it times 1.7. In other words take your cam lift times your rocker ratio to get the lift.
It just depends on how much lift you want. For example the E303 cam has a lift of .498 with 1.6 rockers. Well you could install 1.7 rockers and get a lift of .529.