I know you didn't ask this, but I'd cancel that order and go with someone with a more reasonable minimum. Someone like Murray Mcmurray has a minimum of 15 total. Straight run means they're boxed as they hatch, so an average over a thousand or so birds would be 50/50. If you were getting straight run from me this year, with my hatching mojo, you'd be getting 20+ cockerels
. Those bad boys are not really going to be worth butchering until they're 5ish months old, and unless you're free ranging, they're not cost effective. You've also got to house a bunch of young cockerels that are getting hormonal before the pullets are, and they can make the girl's lives pretty miserable. Just sayin'.
Also, as mentioned, sex links are a one time cross. If you keep a black sex link cockerel and a black sex link pullet, you'll just wind up getting a bunch of birds that are black and black barred, with no regards to gender. You might be thinking of an auto sexing breed like a Rhodebar, but even then they're not always as cut and dried as they should be, due to poor breeding practices.
If you want to breed your own sex links, order some barred Rock hens and a non-barred, non-white rooster. Buff Orpingtons make nice crosses. You're first generation will be sexable at hatch, but after that, again, you lose the magic.