Dropped Ceiling Edge Tile Size Measured in Millimetres
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 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.
Dropped(Suspended)Ceiling End Tile Sizes
The Calculation
 length A of the ceiling if you are using 1200mm ceiling tiles this should be the length direction of these tiles
 length B of the ceiling
Results

Using 600mm tiles in length direction A;
 Number of whole tiles in row. Size of edge tile
 Number of whole tiles in row. Size of edge tile for option two
 Number of whole tiles in row. Size of edge tile
 Number of whole tiles in row. Size of edge tile for option two
 Number of whole tiles in row. Size of edge tile
 Number of whole tiles in row. Size of edge tile for option two
Using 1200mm tiles in length direction A;
Length direction B, since both tile options will be 600mm in this direction;
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