Calculate the size of the dropped ceiling edge tile sizes in millimetres based on running a cross bar or a row of tiles through the centre of the ceiling.
Assuming you want identical sized edge pieces for a direction of length, you have two options for your grid layout.
Ceiling length A = 3.81 metres converted to millimetres = 3810mm
Ceiling Length B = 4.26 metres converted to millimetres = 4260mm
Divide the total length by the size of the tile in this direction:
For tile size of 600mm length A 3810 / 600 = 6.33333. This will give you 6 full tiles in this row.
Multiply the tiles size by 6 = 600mm * 6 = 3600mm
Subtract this amount from the total length:
3810mm - 3600 mm = a left over of 210mm. Divide this amount in two and your end tile size = 105mm for this row. If you end up with an even amount of full tiles in this row then your layout would be based on having a cross bar running through the centre of the ceiling. If you wanted to know what the size would be based on running a row of tiles through the centre (option 2), reduce the amount of full tiles for this row by one and add this to the leftover amount.
Leftover amount of 210mm + 600mm = 810 mm. Divide this amount in two and your end tile size = 405mm and there would be 5 full sized tiles in this row.
It works the same way for using the larger tiles, substituting 600mm by 1200mm in the calculations.