UQL + JSONATA is what you needed I think. For example,
{ "data": [
{ "String": "A", "Number": 80 },
{ "String": "B", "Number": 120 }
] }
parse-json
| jsonata "$map(data,function($v){{ 'S': $v.String, 'N': $v.Number, 'P': $v.Number/$sum(data[*].Number)}})"
