What engine? If it's a small block with a 683 lift cam you don't have that much at the valve anyway because of the losses due to the complex angles of the lifter to the pushrod to the rocker arm. 1.6 rockers will possibly get your lift up to the .683 rating. Big blocks don't have that issue so a .683 lift will change to .728. Like Tony said in the 2nd post all you have to do is first divide what lift you have now by 1.5 and that gives you your cam lobe lift then multiply it by 1.6 (new rocker arm ratio). It's basic math
.683 divided by 1.5 (rocker arm ratio) = .455 cam lobe lift
.455 cam lobe lift times the new rocker arm ratio of 1.6 = .728 lift
You also gain a little duration. Approx. 2-3 degrees at .050"