When I started buying line for my first boat I quickly learned the costliest line was the one I purchased twice because it was too big for the blocks, too small for the hand or too short for the use.
Then I got a little smarter and bought 1' pieces of line types and sizes I thought would work. That allowed me to test the feel, handling and size of the line before I purchased a full length. If I couldn't take an old line off and measure the length, I estimated the length and added plenty of extra. I would rather cut off 10' of extra line than reorder a line that was too short.
I did have the rigging specs for my boat but maybe that spec for the jib sheets did not account for cross sheeting or the halyard spec did not account from running my lines back to the cockpit.
And as jviss says, you also need to account for how you are finishing the ends of the line.
If you are trying to get line on sale, Defender as a spring sale near the end of March. WM always has a big sale around Memorial Day.
I once asked when was the best time to buy sailboat stuff. I got several replies as to when the repeat sales occurred and planned my purchases to catch those sales.
