You could make two parameters, one for selection the field, and another for the multiplier:
I've created one string parameter which is set from the pivot field names field, with an extra option of 'None' so that no multiplier is applied:
Then another for the multiplier intensity:
Then the calculated field:
Then use this field in the viz. Please see the attached workbook.
WeightedAnalysis_v10.2.twbx 63.0 KB