I'm also working up loads for my first 308 and have like you found what works well at 100yds is not the best for 200yds. The 100yd groups are most affected by seating off the lands, while 200yds depends on the amount of powder. Also at 200 yd the best groups occur where the amount of powder has least effect on vertical drop (consistent with so called ladder test). Recently someone posted a wide range ladder test for their 308 which showed a major impact of amount of powder on horizontal spread; across that wide range of powder the vertical was pretty consistent. You did not mention how much work you did to work up the load, so maybe there is more to be learned before writing it off.