To have approximately the same performance as your 70 grain FFg load, reduce the charge by roughly 10%. Since most volume measures are graduated by 5 grain markings, Start at 65 grains. Load development is certainly called for here. Shoot groups of 5 shots off a rest (hand under the forearm that is on the rest). Clean the rifle. Shoot another group at 70 grains. observe if the group is getting smaller or larger. Hopefully smaller. Increase the charge by five grains until the groups start to open up. Reshoot the load that gave you your best group.
The more we know about your load and your rifle, the more likely we will be able to share our best advice. Otherwise, you get our wildest speculation, which may be humorous, but may not be helpful. And up to 20 pages of advice.
What patch material and thickness are you using? What patch lubrication? What are you doing to control the possibility of filling the chambered breech and flash channel with fouling? What do the fired patches look like? Any holes or tears in the material?