I think you are wise testing first at 100 then farther out with the 100 yd established load.
Yes and no.
Fine to DEVELOP a load at 100 yards. Best to FINE TUNE WAYYYYY out there. It is just easier to see the difference at longer range.
I know that there have been a few threads here about "load development at 100 yards". But....I ALWAYS had my own private 1200 yard range, and was always fairly successful in 1K br. THIS YEAR I was restricted to 750 and never did ANY tuning at 1K. I didn't have the greatest of years!! Was it the lack of testing at 1K...OR...I'm getting old?



Who knows.
As far as the OP...I have ALWAYS had better "luck"

Good luck,
Tod