Even if by "optimal" we mean something specific and measurable like "least rolling resistance", there is no such pressure. Minimum rolling resistance depends on the surface. On a smooth surface, like a varnished wooden velodrome track, high pressure is best: the higher the better, with diminishing returns. On a bumpy surface, there is an optimal pressure with negative returns if it is changed either way. (And note, by the way, that an off-road tire with "knobs" constitutes a bumpy surface, even if ridden on a smooth surface.) The pressure depends on how big the bumps are, and the load on the wheel as well as suspension details.
Practically speaking, it can only be experimentally determined; and for that to be valid, we actually need a way to measure the rolling resistance of that specific equipment on that specific surface.