Welcome to the boards, thug. I have a techsights frontsight myself, and I love it. I don't, however, have the peep rear, so I'll just go on general info. Your rear sight 300 aperture actually has a higher center point in relation to the bore than the 200 aperture (think of it as the same as the ramp sight, which rises when you slide the selector forward). Since you sighted in with the 300-yard (taller) rear sight aperture and shot at a shorter range, probably starting off with your front sight too high (understandable, since you just installed it, right?
), you had to lower your front sight to find zero. Sight in again with the 0-200-yard aperture (You'll have to move your front sight up enough to get a sight pic). Get it dialed in at 50, then stretch it to 100 and fine-tune the zero, staying with the 0-200-yard aperture (you should hit a little low at 100 at first). When you're dialed in on the 100-yard target you should hit a little high at 50, dead center point of aim at 100 and a little low at 200. Flip up the 300-yard aperture and you should get hits on 300-yard targets.
Then shoot all the ammo you've got left, take a deep breath and enjoy the scent of cordite and come back and give us a range report!
I'll be waiting for pics on that 300-yard target!