If my diesel engine holds 7L of oil and a 5% dose of Bell Hammer Diesel is about 350ml, adding it directly would exceed the dipstick upper limit. Should I reduce the oil amount first?

 Yes. When adding Bell Hammer Diesel, please adjust the engine oil volume so the final total does not exceed the dipstick upper limit.
If the engine normally holds 7.0L, fill slightly less engine oil first (for example around 6.7L), then add about 350ml of Bell Hammer Diesel. This will bring the total to roughly 7.0L, which fits within the normal range.
Running the oil level just below the upper mark is safe and does not reduce lubrication performance. You can use the product with confidence.