Partial fills quietly wreck your stop and target math

[Amanhasnoname]

Say a scalp is sized at 10 lots with a 3 pip stop and a 4 pip target. The order fills 6 lots at the clicked price and 4 lots two ticks later, because the book at the first level was thin. Now there are two positions, or one averaged position, depending on the platform, with an entry slightly worse than planned. The stop is still where it was, so the distance is a little shorter, and the target is a little farther.

That is the harmless version. The bad one is when only 6 of the 10 lots fill and the remaining 4 stay pending or get cancelled, depending on the fill policy. The trade is now smaller than planned, the stop has the right distance but the wrong money value, and any journal that assumes full size is wrong from the first row.

Check what the execution policy is for your account: fill or kill, immediate or cancel, or return remainder. It sits in the contract specs and is almost never read. Also check whether partial fills show up as separate deals in the history.

If scripts or EAs manage the exits, they must handle the combined position, not just the first deal ticket. Worth testing at the size you actually trade, not at 0.1 lots.

Which fill policy is your account on, and has it ever surprised you?