Well, some of this depends on the pattern of the tartan (if the repeat is such that you can get 4 vertical stretches out of each yard length. You also will HAVE to hemm (as only 2 of the 3 or 4 pieces will have a natural sevage). If you WANT a sevage bottom edge, the amount will STILL only be 2 lengths per yard. Also, do you want a 4 yard, 5, 6, or 8 yard mini-kilt?

Remember, each yard is 36 inches that need to be hemmed if NOT using a sevege. So, 2 yards is 72 inches, 4 is 144 inches (12 feet) and 8 yards is 24 FEET of a hem.

There are a number of factors to take into consideration. In THEORY, you could make an 8 yard mini-kilt with 2 yards of 54 inch wide fabric PROVIDED the sett repeat is a multiple that divides in such a way that all the stripes line up (including the hem).

My personal opinion on hemming . . . I HATE IT!!!!!
I also HATE splicing as well, but if the material is expensive enough, I'd rather splice and pay $170 for 2 yards rare Strome for one 4 yard box than $340 to have 2 identical kilts, but not have to splice. (that is for the material itself, PLUS my labor to make the kilts)
BUT, for $12/yard fabric, it IS worth $24 for me NOT to have to splice!! Then I can use the other half for other stuff.

Note, some kiltmakers even REQUIRE you to buy a straight length of material for any kilt they make and either provide the other half to you or seek for you to have them make 2 kilts. This is all to avoid a splice.
The better ones will splice for you, I KNOW Barb (in her book) and Matt (how 2 yards of fabric become a 4 yard kilt) do.