So, I am not new to spey rods and have quite a few, however I grew up in Scotland but moved 20+ years ago over here. My father, who is still in Scotland gave me a few of his rods with the appropriate spey lines to go with them. I love these things, older slow.. slow action rods but mostly because they have meaning to me. To avoid the TLDR;, I want to put modern lines on them. Although it's fun casting spey lines, it's a lot less work to cast scandi heads or skagit heads.
My issue. These are AFTM 10/11 but if I go with the recommended heads for a 10/11 they totally overload.. I mean it's not even close. If I line down from there I get the same thing. I have a 6wt (Echo Tr 12'6) that takes 540gr skagit heads but the last time I tried putting a 625gr on one of the '14 10/11wt rods it felt like i was casting a noodle and couldn't successfully cast it at all.
Seems odd to ask but is there some old -> new or uk -> us conversion for AFTM (Yes I am aware this is an american standard)?