As stated, every gun is different. What works in one gun may not work in another gun.
First, move your target up to about 30 yards and don’t move it back until you find an accurate load. 100 yards is too far to work up a round ball load
i might start with 60 gr of powder, shoot 3 times. Then 65, skip 70 since you already did that, then try 75, 80, 85, and 90. Would not go beyond 90 and hopefully you can find accuracy with less than that