For products that are not covered by UL60950-1 safety standard, to determine the electrical clearances the designers normally consult with IPC-2221. It is widely accepted throughout the world as a generic PCB design standard for commercial and industrial applications. The Table 6.1 of IPC-2221A specifies minimum required conductor clearances as a function of voltage, elevation level and the coating. Since their introduction in 1998, these numbers were never revised. IPC just added the values in inches in revision A and left them unchanged in rev.B.
Of course, it is always desirable to maximize whenever possible the distance between tracks on individual layers to minimize the possibility of electric breakdown and reduce parasitic capacitance. However, because of usual shortage of space on a PCB, spreading out the traces and components more than it is really necessary may not be feasible. From a technical standpoint, IPC stepwise clearance limits are mostly baseless. For example, there is no reason whatsoever, why you need 2.5mm for 301V, while for 300V you can use 1.25mm. A recent IPC-9592 standard for power conversion circuits originally provided linear functional spacing requirements: SPACING (mm) = 0.6+Vpeak×0.005. Linear function of course makes more sense. However, in most cases the above formula resulted in even higher spacings and in grossly over-designed circuit board. Later revisions of this document returned to a sort of step function and relaxed the requirements at the low end: 0.13mm for V