I'm no expert, but based on my limited experience, and equally limited understanding, of both full-size and miniature loco springing, my response would be as follows based on experiments with the leading pony truck on my loco:

Firstly, do you intend your trailing truck to take any component of the loco weight under normal conditions? If so, do you intend to provide any secondary springing between the trailing truck frame and the loco chassis? If not, is the trailing truck frame to be restrained to prevent any vertical movement?

If the trailing truck is to have secondary springs, then the softer primary springing on the axle-boxes should be set around mid-height to account for minor variations in track alignment, with the stronger secondary springing taking care of larger humps and hollows beyond the movement available in the primary springs. The secondary springs should be adjusted to hold the trailing truck frame in the correct vertical position relative to the loco chassis when the primary springs on the axle-boxes are at approximately mid-height.

If there is no secondary springing, and the trailing truck frame is fully restrained vertically, the axle-box springs should be set at mid-height to allow the axle to follow track irregularities. If the trailing truck frame is free to drop to allow the wheels to follow hollows, but not to lift to follow humps, the the primary springs should be set so the axle is at the lowest position with the loco standing on level track.

The free length of springs, and strength of springs, will need to be adjusted to suit the individual scenarios described above.

Of course, this cannot be properly set up until the loco is at it's running weight, as up until then it's all guesswork. Your design should provide some means of adjusting the spring settings once the loco is completed. This may involve replacement springs of varying length and strength on a trial and error basis, or adjusting screws on the spring supports.

Others with more extensive experience will no doubt provide more detailed advice which could contradict what i've said above. If so, i suggest you take their advice.