That's a really good idea. I guess the long side could also be the " porch",if you are on flat land.:mellow:
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I always just stretch my arms out as far as they go and that's how long my linee are. I never measured them but I'd guess 6 to 8 ft. They always seem to be long enough even with the hiking pole porch setup. That's always been my barometer...long enough to set up the porch.
I have 4 lines (of 2mm dyneema) permanently attached to my tarp (overhand knot on the end then larksheaded onto shockcord loops) which are each 2m (~6.5ft) long, which to date has been plenty long enough. I do carry extra cordage so if needs be I could always extend the lengths.
I start with a 10' piece and splice eyes on each end, so they prolly end up 9'6"-ish.