Ogri the trog, et al,
Welcome to the discussions... new guys are always welcome.... they often bring a new perspective...or even if an old point, we all rethink the issue together...which is good IMHO.
Good point on shorting a tarp with a pebble and a loop hitch... Personally I try to use gear within its design parameters.... very light gear, silnyl, spinnyl etc are prone to damage if not careful.... additionally, if shortening by the method you suggested, the sides may not tension well and you may have to put up with a bit of noise/flapping... extra tie outs may help.... but it is a technique worth remembering if one really wants to use a couple trees that are too close.
Ounces to Grams.
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