The 5/16" will do fine with your nutshell I even would opt for the 1/4" as your dinghy is very light. I pull my dingy (14' Achilles with 25HP OB and 10 gallons of gas and bow-bag with 10lbs anchor and pump, and 4 PFD's) across Lake Michigan all the time using a double 5/16" painter about 30' behind the boat. I also have the loop end at the boat and I have an extra second 1/4" safety line.
I make fast running the bitter end of the painter thru both side bow "D" rings and than back to the boat knoting a second loop, I put both loops over/thru one of my aft cleats or use both SB and Port cleat. This enables the dinghy to slide sideways keeping force off the center "safety" line which is connected to the center tow/bow ring on the dinghy.
I must use this set up as I can't lift the 250lbs dinghy and/or the 140lbs OB on board (I should get davits, but the expense keeps me from it).
This seems to be working fine although I haven't used this set-up in 5 ft or more waves and if caught with these higher waves I would double my tow length if not triple it.
Last but not least never assume the dinghy is there, i.e. visually check at regular intervals.
S/V "Sailmates" 1973 IRWIN 32 Classic
I want to live and sail forever, so far so good[/SIGPIC]