I think it always better to have longer lines then needed. For the summer my lines are about 15' and in the fall I'll add another 15'.
If you've using the small donut shaped rope floats, insert a lenght of 3/4" pvc pipe and wrap the excess in a figure 8 pattern around both ends of the pipe. Finish the line with a clove hitch to secure the line.
Another method is to use a a 12"-15" section of a pool noodle (the thicker ones work best). Slice the end 1/3 of the way towards the center. Attach your line to the middle and wrap the line around the middle until you get the length needed. Then take the line slide it into the slot. Use a thick rubber band or make a bungee loop using 1/8" cord. Slice the rubber band/bungee ring over the slotted end of the float. This keeps the line from winding off the section of pool noodle.
If you're crabbing flat/slow moving water and depending on the type of boat you're using -- 4' of line longer than the water depth should be fine. If you crabbing in a strong current, I'd add additional line. I've had floats pulled under by the current -- you might even want to use a larger float.
It helps to mark you line every 5', some use a sharpie other use several wraps to colored fishing line. This way use can easily tell who much line you have out. Nothing worst than having 25' line out in 10' of water.
Just fyi, I believe the regs require you to mark your floats; use a wide sharpie and put your name, initials or boat number on each one.
Good luck crabbing, hoping to hit the Wye in the next couple of days.
Dan aka 'pour-me'
Four Winns Quest 217
Havre de Grace, Md