Hi partner,
I would recommend you create a simple filed to represent percentage, and use business rule to calculate the manual discount field.
The disadvantage of using calculated field is that calculated field can only be updated when the record is saved. So when the Quote Line record is saved for the first time, the calculated field is updated, and the manual discount field is also updated. At this time, we need to save the record a second time to get the correct Extended Amount.
1. Create a floating point number field for Quote Line entity. Minimum Value is 0 and Maximum Value is 1.

2. Add this field to Quote Line form, then create a business rule as follow. Select the Scope to Entity.

IF
percentage discount contains data
THEN
Set Manual Discount to Amount * percentage discount
In this way, whenever user enter a percentage decimal, the Manual Discount will be the percentage decimal * Amount.