How Price Calculation in Updation of Product Variations Works

Case 1 : I want to increase / decrease all ‘Large’ item prices by X

Keep ‘Base Price’ as blank and set the differential price of ‘Large’ attribute to 2. Do not forget to select other attributes & their options which are already the part of that product. You can keep the differential price value as empty.

Case 2 : I want to calculate & apply new set of prices

Set a base price & differential prices as described here.