Convert Account Value with the Rate Available to the Decision
By DX Research Group · · Market data
A two-rate fixture separates native-currency performance from reporting-currency movement.
Use a recorded, directionally explicit conversion rate to express account value in a reporting currency. Preserve the native-currency ledger and the conversion separately. An agent whose limit is stated in dollars may receive a different budget after an exchange-rate move, even when its holdings remain unchanged. We would bind the conversion observation to that decision's valuation policy.
The ECB's reference-rate page states that its rates quote currencies against the euro and serve informational purposes. A reference-rate valuation and an executable conversion quote therefore support different claims.
The account is unchanged while its dollar value moves
Assume a fictional account holding €1,000 cash, no positions, no deposits, and no trading costs. The rate available at the decision is 1.10 USD per EUR. A later rate is 1.20 USD per EUR. These are illustrative values rather than actual ECB observations.
| Valuation | Calculation | Dollar account value |
|---|---|---|
| Original decision | €1,000 × 1.10 USD/EUR | $1,100 |
| Later reporting | €1,000 × 1.20 USD/EUR | $1,200 |
The reported dollar value rises by $100 while the native account remains €1,000. Dollar return is 1200 / 1100 - 1 ≈ 9.09%, entirely from conversion in this fixture. Treating that change as trading profit would attribute currency movement to an agent that made no trade.
A 10% dollar exposure cap at the original decision gives $110. Replacing its conversion with the later rate yields $120, altering the historical authorization by $10. Preserve the rate used for the decision even when a later report adopts the new rate.
Store the direction before multiplying
Write conversion units explicitly. USD per EUR means multiplying euro balances by the rate. EUR per USD means dividing those balances to obtain dollars. For a cross conversion, preserve each leg and its observation so a derived rate can be reproduced.
A crypto collateral asset also requires its own valuation convention. A stablecoin balance can be reported at a contractual or accounting parity assumption, or at an observed conversion quote. State which convention applies, and record any departure from parity rather than hiding it inside a generic currency code.
Keep reporting and execution costs distinct
An informational daily reference can support a declared reporting policy. A real currency conversion additionally has execution spread, fees, and settlement mechanics. A backtest using the reference rate should label those omitted costs or model them under explicit assumptions.
A proposed valuation fixture would replay both rates, invert the stored unit deliberately, and remove one cross-rate leg. Expected checks include correct original budget, a separate conversion component in return reporting, and an unavailable conversion when a required leg is missing.
Our numeric-mandate note connects the budget to a user's stated unit. Our portfolio-reconciliation note covers balances before new exposure. This calculation establishes transparent currency accounting. It supplies no independent evidence of trading skill, and a real account result still needs its cash flows, venue receipts, and valuation convention.