Percentage fee truncation underpays the treasury

calculateFee() truncates the percentage fee before comparing it with flatMinFee:

return Math.max((executionPrice * feeBps) / BPS_DENOMINATOR, flatMinFee);

Integer division discards the remainder, so the fee actually charged is below the configured percentage whenever executionPrice * feeBps is not a multiple of BPS_DENOMINATOR. With the deployed feeBps = 500, an execution price of 1.000001 USDC has an exact 5% fee of 50,000.05 base units; the contract charges 50,000, while the smallest representable fee not below 5% is 50,001.

At 5%, 19 of every 20 base-unit price residues carry a non-zero remainder, and the discarded fraction reaches 0.95 base units. The treasury therefore receives up to one base unit (0.000001 USDC) less than the configured rate on almost every settlement, and _executePayment pays that difference to the seller instead — quote.safeTransfer(seller, executionPrice - fee) is computed from the same truncated fee.

The treasury is under-paid relative to the configured rate on almost every settlement, and the difference is paid to the seller instead. The shortfall is at most one quote base unit (0.000001 USDC) per settlement, so the loss is an accounting leak rather than a material one — but it is systematic and always in the same direction.

Round the percentage fee toward its recipient rather than truncating: take the ceiling of executionPrice * feeBps / BPS_DENOMINATOR before the Math.max against flatMinFee.

Keep the multiplication checked. Applying Math.ceilDiv to the checked product preserves the existing overflow revert. Replacing the whole expression with a 512-bit helper such as Math.mulDiv(..., Math.Rounding.Ceil) computes the same fee but removes that revert, retiring the guard test_calculateFee_overflowReverts exists to prove.

Three existing tests encode the truncating formula and need updating with the same rounding direction:

calculateFee now rounds toward the treasury using Math.ceilDiv over the checked product, so the overflow revert on executionPrice * feeBps is preserved — Math.mulDiv would have removed it. Verified at audit-v2.

• src/PlakxioSettlement.sol#L569-L572 at commit 5c38893