You cannot say how much line you need out, you can load a rod with 20ft of line, also you can load the rod with 75ft of line out. The 30ft of line that Alanc maybe on about, is what is weighed to get the AFTM number for a given line. Also, alot of manufacturers use different colours along the line to show where the head ends and where the running line begins.
As an example, I have a Vision GT4 Catapult rod, 9ft for a 6# line and I have 3 floating lines, say a Greys platinum, A Pitsford Pirate special and Loop Opti stillwater line, then they would all load the rod, you may get 45 ft of one line 30ft of another and 50ft on another outside of the tip to get the same feeling. The only way to find out is to play wiith the rod and find out what line legnth outside of the tip works best for you
No idea if any of the above helps at all?!
Chris