I'm far from a pro---
but the whole line weight designation issue for DH rods irritates me.
Ask around, search the pages, or contact the manufacturer to get grain range information on the rod. For lines, I simply refuse to buy spey lines that don't have grain weights clearly on the label; it's a disservice to the customer to leave that off the packaging.
For myself, when buying a line, I ignore the line weight rating and find out the grain weight of the HEAD. If I'm buying a DT line, I do my best to find out what the first thirty feet of line REALLY weighs (varies considerably from AFTMA standards sometimes), and what the anticipated working length of the line weighs. Some shops, particularly Poppy at Red Shed (the MJC who just posted on this, hi Poppy!
), are extremely helpful with this.
A helpful rule of thumb in balancing tackle is that the longer the line being cast, the closer it should be to the high end of the grain window. While "your mileage may vary" as they say here in the pages, a useful starting point is to consider short scando heads near the bottom of the grain window, skagit heads (not including tips) slightly higher, then up through short, mid belly and finally longbelly lines which are usually balanced at or near the high end of the rod's grain window. There's widely differing opinions on this, particularly in Skagit world.
I break this rule regularly...but it is a good starting point for discussion.
casting characteristics of the rod and the caster's own technique (or lack thereof) will influence this greatly. If a shared opinion develops in the pages, it's probably been learned through hard experience, and I wouldn't worry about following it.
hope this helps, anybody wants to correct me on this please feel free to jump in.