Pick the form, drop the photo or signature. It is drawn at the stated pixel size, and the JPEG quality is searched until the file lands inside the KB range at the best quality that fits. Every result says pass or fail against the numbers.
How the size is reached
The picture is drawn at the preset's pixel size straight from your original — cropped to fill or padded with white, your choice. The JPEG is then encoded again and again, each time measuring the real file, until the highest quality that stays under the maximum is found. Nothing is estimated.
If even quality 40 is over the maximum, the page says so and offers lower quality with a preview. If even the highest quality is under the minimum — common with a clean signature — the page explains rather than padding the file with junk bytes to cheat the check.
Portals disagree about whether a kilobyte is 1,000 or 1,024 bytes, so the maximum is checked as 1,000 and the minimum as 1,024: a file that passes here passes under either reading.
Where the presets come from
Every preset names its source and the date it was read. Presets read from a third-party summary rather than the official notice are marked “confirm on the official portal”, because exam notices change between cycles. The page claims the file meets the stated numbers; it cannot promise a portal will accept it.
DPI, where a form states one, is written into the JPEG's header as a label. It does not change the pixels: what a portal actually measures is the pixel size and the file size.