When the surrounding flaps are folded up, the base of the box will have dimensions
by
, and the box will have a height of
. So the box has volume, as a function of
,

I don't know what technology is available to you, but we can determine an exact value for
that maximizes the volume by using calculus.
Differentiating
with respect to
gives

and setting this equal to 0 gives two critical points at

For the larger critical point we would get a negative volume, so we ignore that one. Then the largest volume would be about 168.5 cubic in.