The holiday period contributed a lot to that average. I also lived with other people for the whole year (uni housing, with family and now with flatmates), so meal times would count towards socialising. And those coffee breaks and quick chats here and there add up to a lot!
I try to note start and end times of activities. At work, or whenever I'm doing something on my laptop I would fill the spreadsheet as soon as I finish or take a break from an activity (eg finished reading a paper, going on a coffee break now). When I'm doing something more varied I would note start/end times via google sheets app on my phone or just remember them and fill them once I have access to my laptop. Sometimes, I would use data from my fitbit (to see when I started/stopped walking) or my search history (to see changes between working vs looking at silly things on the internet). The categories were broad enough that logging the data is not too time-consuming and often they could be added as big time-blocks of a single activity.
I probably used productivity too carelessly in my article - I meant it more as 'doing things that make me happy/help me develop as a person' as opposed to 'doing things that feel nice and easy now but I'd rather do them less'. I consider hobbies, reading books, going on walks and things like that as productive and tried to reduce time I spent mindlessly scrolling social media.
I'm also doing my PhD so by trying to increase my at-work productivity I'm trying to make sure that I don't waste too much of taxpayer's money.
But I agree that optimising productivity too much could be anxiety-inducing and I was trying to be mindful of that while looking at my data and deciding what to change in my life based on it.