266. The net gain contemplated in section 265 is computed:(a) by determining the amount by which the aggregate of all the taxpayer’s gains for the year from the disposition of precious property, except prescribed cultural property contemplated in section 232, exceeds the aggregate of his losses for the year from the disposition of precious property; and
(b) by deducting from the amount so obtained such portion as the taxpayer may claim of his precious property losses for the seven preceding taxation years and the three following taxation years.