XPath for less than or equal to a percentage?

I'm using this code right now (XPath) with WP All Import for Wordpress to import large CSV files.

Here is the plugin doc:

http://ift.tt/1z5Wowr

/node[price[1] < old_price]

But I want that price is only 80% or less of old_price. How can I code this? Is there a simple code, or I have to write a large new code?

via Chebli Mohamed