Calculation of loss is dependant on the method used to express the loss.
Usually, in process costing, each process is considered seperate. And so, the opening WIP represent the stock available for the process. Normal loss is the loss of units in a particular process. Though normal loss is already accounted for in the opening WIP, that would be of earlier process. Hence normal loss may be computed on (Op. WIP + Inputs).
If there is no specific mention in the question, the loss is to be calculated based on gross input.
Normal Loss units = Gross Input x Expected percentage of normal Loss.
For any mistakes, I may stand corrected.