Assignment is the moment a tracking system either holds together or quietly comes apart. An option position ends and a stock position begins, and the two are the same trade — but almost nothing in the tooling chain knows that.

Your broker records a closed option and a share purchase. Your spreadsheet has an options tab and, if you are organised, a shares tab. The relationship between them exists only in your head, and that is where wheel records go to die.

What actually needs to survive the transition

Three pieces of information have to make it across the boundary, and losing any one of them breaks a different downstream figure.

The first is covered in cost basis after assignment. The second is the campaign concept. The third is the one people forget entirely, and it matters for holding period: your shares were acquired on the assignment date, not when you sold the put.

It happens in both directions

Assignment on a put brings shares in. Being called away on a covered call sends them out. Both are hand-offs, and the second one is where a campaign closes.

The call-away side has its own requirement: you have to capture the effective basis at that moment, because once the shares are gone the running basis column has nothing left to point at. If you do not record it, the cycle's result becomes unrecoverable — you know what you sold at and not what it cost.

The share lot is the right object

The structure that works is a share lot record: quantity, acquisition date, raw purchase price, and a running effective basis that every subsequent option credit modifies.

Option trades then reference the lot they relate to. A covered call written against lot #3 posts its credit as a basis reduction on lot #3, and when lot #3 is called away the campaign's realised result is computable in one step.

This is genuinely harder than a flat table, and it is exactly the boundary described in where spreadsheets stop working. For a single lot it is manageable by hand; for several lots at different bases on the same ticker it stops being manageable quickly.

Several lots on one ticker

Assigned twice on the same stock at different strikes and you now hold two lots at different effective bases. Every covered call you write has to be attributed to one of them, and when shares are sold you have to know which lot went.

Brokers apply a default disposal method — usually FIFO — and it affects both your realised result and your tax outcome. Two practical consequences: know what your broker's default is, and if you track lots yourself, use the same method or your figures will not reconcile to the 1099-B.

What can go wrong

Treating assignment as two unrelated events. The default behaviour of almost every tool, and the reason wheel records fragment.

Losing the basis when shares are called away. Capture it at the moment of the hand-off, not afterwards.

Allocating premium per position on a partial assignment. It belongs to the contracts that were actually assigned.

Using a different lot method from your broker. Your figures will never reconcile, and you will assume you made an error somewhere else.