[MegaTrain]

The most efficient way to make sure that your construction queues are going to be used to full capacity is to design your ships with the largest cost (typically minerals) just under a multiple of the build speed.

Lets say your spaceyards are building at 3000 per turn.
Good ship designs: 3000 (1 turn), 5900 (2 turns), 8950 (3 turns), etc
Bad ship designs: 3100 (2 turns), 6250 (3 turns), 9400 (4 turns).
I sometimes even designs variations of the same ship for different speed spaceyards.