It's very simple, if you miss more than you hit then you either need to move closer or get bigger steel. There are times when the wind varies in intensity enough so that you'll hit left, next shot right, and luck one in there once in awhile, which can be extremely frustrating! There are also times when the wind switches or lets off with bullet mid air which gets confusing. The farther away the bullet gets away from the muzzle all this is compounded. Also the vertical component is affected depending which direction the wind is coming from.
I had a 375CT which is a cartridge having few rivals when it comes to wind. On one windy occasion a friend with his 6BR had a hard time hitting a 1 foot wide plate at 600Y. I was shooting that 375 at 1000Y and getting good groups on a slightly larger steel. Good for me but he might as well have left. Another windy day, one of those days that started with little wind and was blowing 25-30 MPH a few hours later. What was relatively easy to hit earlier with the 375 at ELR distances became a total waste of ammo by the time we threw in the towel, we should have left a good half hour earlier.
With a 6.5 creed I'd pack up when it gets to 15mph or so and the reason is mostly because you won't be learning much by that point.