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Lower the better. Fewer molecules to push out the way. But I'm not sure at what point it's low enough to make a noticeable difference in a tt. For reference, last night was 1001mbar and for Wiggin's hour record attempt, where everyone said the air pressure was "high", it was 1036mbar.
https://www.shopforwatts.co.uk/blogs/news/dowsett-hour-record-take-2
http://www.cas.manchester.ac.uk/restools/whitworth/data/index.html
I did this too, apart from the wee in a field. I then drove halfway to Chelford before binning it off and heading home due to the weather.