"He who marches out of step hears another drum." ~ Ken Kesey
Whenever possible, I kick back the conversion to them. I ask them to tell their designer that if printed in spot colors, the colors need to be spelled out with the standard Pantone equivalents. Works most of the time when they have a "real" art person creating their art.In other instances, I'll do the "match by eye with the book" method, but I'll ask for a hard copy version that they have signed off for in the past for the comparison.. When it's a judgement call, whenever possible, I'll make the final decision theirs.
pantone color bridge for cmyk!pierre
I use photoshop color picker. Have not had any complaints that come to mind. I never promise an exact match and usually tell them the pantone color I will be using unless they tell me otherwise.