It's difficult to establish a specific formula for changing pH because of the unknown buffering effects of various growing media. Peat-based soils naturally buffer to 6.0 - 6.5 but can be overwhelmed.
My approach is to change the pH of the water. 1 ml of pH Down (citric + phosphoric acid) in 2 gallons (about 8 liter) will bring 8.3 down to 6.0 (pure water has no buffering). But what that actually does in the soil is unquantified - I'm relying on the buffering of the peat to keep it about there.
FWIW, I've successfully grown peppers in soils as acidic as 4.5. Erring on the acid side is much better than erring on the alkaline side.
How are you testing?
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