For the lines an account already holds, the norm's median spend on each line less the account's spend, floored at zero, summed. The upsell half of the share of wallet gap; whitespace is the breadth half. An account with every expected line and a low share has a depth gap and no whitespace.
Whitespace value plus depth gap equals the share of wallet gap, when both use the same norm.
The account that holds everything thinly is called full by the grid and found by the depth gap.
For each line the account already buys, the norm's median spend on that line, scaled to the account's size, minus the account's spend, floored at zero.
An account buys fixings at $4,000 a year where similar accounts spend $11,000. The depth gap on that line is $7,000. It is separate from the lines the account does not buy at all.
Only unbought lines counted as opportunity. Underbought lines are often worth more and are an easier conversation.