Test curves

Hi folks, After an enforced break of several years, I'm getting back into this fishing lark. When I was a lad there used to be a rule of thumb to calculate the right maximum casting weight for a rod. (They never used to be marked).
It was something like:- Each pound of weight needed to bend the rod through a smooth 90 degree curve was the number of ounces of casting weight.
Does anybody remember this? Does it still apply? How does line strength fit it to this? Can it be applied to unmarked fly rods to fly lines?