How does the grain reduction calculation work ?

For example, take an Agema which has a base grain usage of 4 per unit. Then apply reductions eg demeter, grain reduction via academy and coalition capital discipline. When i do the maths I get a lower number than shown when looking at the details for that unit. Specifically - demeter at L20; academy reduction at L61 and capital discipline bonus at 10%. My maths says this should be 0.1544 per unit but the game shows 0.18 per unit.

Not a huge difference I know ... but I like my ducks in a row :)

EDIT : ok - another puzzlement - where can I find the details of all the different things that affect resources / eg I remember some things like a bonus once all articles were collected (or something similar). The 'standard' ones eg Pan / Demeter etc are easy to factor in but there were others from earlier in the game but I can't remember them.