I believe there is no such thing as "upside down" with groove/flat edges as I recall.99% of people install the boards upside down.
Anyways I'm not installing decking (yet! / this project..... Later for the garden room next year.... But I'll go composite for sure)
Moved over 4 tonnes of grit (/sharp) sand and cement ( 171bags) into position ready for mixing. I'll be doing a 5:1 mix with a little plasticiser in it.
Ive bought some prebagged larson primer rather than mixing my own primer with SBR.