Score starts at 0 and can never go down. You always keep what you earn. Score is calculated internally by Dance Points (DP) and then normalized to a maximum score of 100,000,000. All scoring is per-note, not per-row. Judgements, however, are per row (in foot game modes). This means that a Jump is counted as a single action with two Notes. If you miss one, you miss both.
https://docs.google.com/spreadsheets/d/1d6RxvaiXPSw0J-ZGJPHFHX7IH3JxoaemrQboKKgO9a0/edit#gid=0
| MIGS | SM6 |
---------+------+-----+
Flawless | 3 | 9 |
Perfect | 2 | 6 |
Great | 1 | 3 |
Good | 0 | 2 |
Bad | -4 | 1 |
Miss | -8 | 0 |
OK | 6 | 9 |
NG | 0 | 0 |
Mine-OK | 0 | 9 |
Mine-NG | -8 | 0 |
---------+------+-----+