Please help how I can change the number format of a list subset to a percentage.
You should be able to change the format of any line item in the blueprint mode unless I'm not understanding your question.
You can even change the format on the line item that does the COLLECT().
IF your question is how to change the format of a line item from number to percentages - see below
If your question is how to change the format of a line item subset from number to percentages - Again the answer remains same however if your line item subset has one item which is pulling percentages and the rest pulling as numbers, there is no direct way of doing it.
You will have to pull the numbers as is and then created multiple modules to fix that. Let me know if this is what you are looking for I might be able to create something for you