एवं त्रिभागोनो रथानामावापः कार्यः ॥ कZ_१०.५.२२ ॥
evaṃ tribhāgono rathānāmāvāpaḥ kāryaḥ
In this manner, the ‘filling’ (āvāpa) of chariots should be done so that one-third is kept back (i.e., not added to the side-divisions).
As a ratio instruction: allocate two-thirds outward, retain one-third for the center (or reserve/anchor).