OK, typically if using a dry line the overall leader length for moving water systems should be in the region of 12ft overall, may be more for certain situations and less if fishing say a small creek with little room to work with either with a long rod or a long leader.
Rarely will by droppers distances be less than 24 ins more like 30 to 36.
But there may be differences which would amount to the mid dropper being 4ft from the top dropper or the tail fly 4ft from the mid.
If l have the mid dropper 4ft from the top dropper then l know this fly and the tail fly will remain deeper in the water as l work the top dropper at long range back toward me, that also adds a further anchor effect.
If l have say the tail fly 4ft from the mid then l know that the fly has a little more freedom to animate, often this is what l would choose to use for more upstream to across techniques.