So overall I'd say LG long range artillery fire is at least 5 times better than it should be.
If you dropped long range artillery fire by 80%, the result would be that it would be pretty much totally ineffective. Halting for artillery preparation would be the height of folly. Regimental guns would be detrimental, as they wouldn't have enough effect to bother with and they'd slow down the regiment.
Small changes here have large results on the battlefield. You could still ignore enemy artillery with a 50% reduction. 20% would be more reasonable.
However, I don't believe it's the strength that's the problem here. Long range fire seems quite accurate to me, especially against moving troops. I'm not sure how accuracy is calculated, but I know individual guns are tracked as well as individual cannonballs, and we see dirt sprayed up where the balls hit. Even the smallest change to the accuracy of the individual guns here would have major effects, so caution is needed. If the accuracy is adjusted, the strength doesn't need to be, especially if every round doesn't result on a hit on a stationary target after it's gotten the first hit.
The other part of this equation is the morale effects on the target and what actions they should take under various conditions.
Long range counter-battery fire hits more than just the physical cannons. It also takes out crew and support troops. You can render a cannon hors-de-combat just as easily by killing the crews as destroying the physical cannon, although the only effect we see is destroyed cannons, which I would consider an acceptable abstraction. An accuracy reduction would make a difference here as well.
Hook