Start a ceiling grid from one corner and you finish with a full tile on one wall and a 60 mm sliver on the other. This centres the grid so both borders match, then counts every component you have to order.
Notes
Why borders are centred
A ceiling is seen whole, from below, with the eye running to the edges. Unequal borders read as a mistake from the doorway. Centring costs nothing and takes one extra calculation.
When to shift the grid off centre
If centring leaves borders narrower than about a third of a tile, drop one tile from the run and re-centre. Two 300 mm borders look intentional; two 40 mm ones look like a cutting error.
Hangers
Main runners are normally supported at regular intervals along their length and near each end. The spacing depends on the system and the load above — set it from your manufacturer, not from a calculator.
What this does not do
It lays out a rectangular ceiling. Bulkheads, columns, light fittings and air terminals all displace tiles and are counted separately.