I seen in another link that the minutes spent watching a programme are decided by the minutes in the programme
If there are fifty minutes in the programme and you watched for five minutes, you are counted as 1/10 of a viewer.
This link gives answers to various questions and says about the calculation,
"Programme audiences are calculated by averaging the audience of all minutes covered by the programme transmission, from the start-time to the end-time of the programme. This calculation excludes any embedded commercial breaks and promotions."
That ties in with your being 1/10 of a viewer.
The FAQs also tell you how to find viewing figures.
https://www.barb.co.uk/frequently-asked-questions/