VEGA FIELD GUIDE · 19 · RISK & WORKFLOW
Position Sizing, Liquidity, and Assignment Risk
A clever payoff can still become a poor trade through oversized risk, wide quotes, or a surprise assignment. Position mechanics deserve the same attention as the market view.
Size from a defined loss scenario and total portfolio risk. Premium is one input in that budget.
Bid-ask width, quoted size, open interest, and volume provide different pieces of the liquidity picture.
Short American-style equity options can be assigned on any eligible exercise day.
Begin with a loss budget
Choose a dollar amount the portfolio can absorb under a realistic adverse scenario. For a defined-risk spread, maximum loss is a useful hard boundary. For long options, the entire premium can disappear. For stock-backed and uncovered positions, stress losses can be much larger than the opening credit.
Portfolio context matters. Five positions tied to semiconductor volatility can behave like one large position on a difficult day. Aggregate delta, vega, event dates, and sector concentration before counting each trade as independent.
floor(trade risk budget / stressed loss per contract)- Defined loss$500
- Spread and fees$35
- Assignment reserve$8,000
Worked example: size the spread
A portfolio has $50,000 in capital and a per-trade risk budget of 1%, or $500. A proposed vertical spread has a $180 maximum loss per contract after estimated opening and closing fees.
Two contracts place $360 at contractual risk. Three contracts place $540 at risk and exceed the budget. The two-contract size leaves $140 of room for worse execution or small adjustments. Correlated positions can justify an even smaller allocation.
| Contracts | Maximum modeled loss | Within $500 budget |
|---|---|---|
| 1 | $180 | Yes |
| 2 | $360 | Yes |
| 3 | $540 | No |
Liquidity has several dimensions
The bid-ask spread is the immediate cost visible on the screen. Quoted size shows how much is displayed at those prices. Volume reports today’s trading, while open interest reports outstanding contracts after the prior clearing cycle. A contract can score well on one measure and poorly on another.
Multi-leg structures compound execution friction. Midpoint marks can make a backtest or payoff preview look generous. Model an entry near the ask for purchased legs and near the bid for sold legs, then test a more favorable limit as a separate case.
A $0.10 spread on a $0.30 option is one-third of the quoted midpoint. Percentage friction can become the main character in low-premium contracts.
Assignment changes the position
When a short equity call is assigned, shares must be delivered at the strike. When a short equity put is assigned, shares must be purchased. Assignment can occur before expiration for American-style contracts. A hedge leg elsewhere in the account requires its own exercise or closing action to preserve the intended spread.
The remaining legs may carry different risk, margin, and tax consequences after assignment. Confirm broker notification timing, exercise cutoffs, auto-exercise rules, and liquidation policies. These details are dull right up until they are fascinating.
The final pre-trade pass
- Record maximum loss, stressed loss, and expected shortfall.
- Check spreads, sizes, volume, and open interest for every leg.
- Review earnings, dividends, splits, mergers, and expiration settlement.
- Confirm buying power under a gap and a volatility spike.
- Write the exit, roll, and assignment response before entry.
Start with usable markets
Compare expirations, quote quality, activity, and available strikes before sizing a structure.