That's exactly how you would do it
User input is via a parameter
Calculated field takes the parameter and multiplies it
View is based on the calculated field to show the result as a line on the graph.
Yep. Create parameter as a float or integer. Feed that into the calculated field. Use the calculated field in the viz. Display the parameter that will look just like a filter container. And place it wherever you want on the dashboard.
Lots of literature out there on this: