The method to do this is the formula pulldata() with a CSV lookup table to get the values.
I don't believe this is supported in the web designer, you need to use connect.
You could build the survey in web designer then download into connect to add the formula and CSV, but once you do you will need to continue to use Connect.
Personally I would recommend just learning Connect if you're going to be working in this space a lot. There's a lot of things you'll learn about good survey design that's not at the front of the web designer. If you're not, and you're confident this is the only survey you'll ever make, I'd stay in web designer, keep the survey simple, and do calculations like this AFTER the data has been collected. E.g. using pro, field calculator, joins...
