Just get a chunk of cheap vinyl hose, 8mm i.d. is good, and lay it out where you want your line to run, and trim it to length. When you're done measure it and you will have the minimum length you need.
Interesting anecdote, during WWII, the design length of the V-2 rocket was determined by towing a long log from the rocket factory to the proposed launch site. Whenever an obstacle presented, the log was shortened to get through. When it finally reached the launch site, it was measured and that became the length of the rocket.