Dalton,

Best knot is a taut-line hitch. http://www.netknots.com/rope_knots/tautline-hitch

There should be absolutely no slack in the ropes and that knot will slide completely tight and will not slip. It's easy to tie with a bit of practice (similar to two half hitches).

Don't have a maximum wind speed, but tie downs should hold at 40 MPH. If you have to deal with a tail wind, cock the nosewheel to one side when you park the airplane. That will deflect the rudder to the stop and helps keep it from banging back and forth if you don't have an external gust lock for the rudder.

Warmest regards,
Rick