No, there is no way to designate a % of calls to do a screen recording on.
The article you linked is just asking you to guess what % of recordings will be retained based on the Policy settings. It's not real clear on that, but there's not much of another way to state it.
------------------------------
George Ganahl GCP (PureCloud) ICCE CCXP
Principal Technology Consultant
Genesys
------------------------------