I think the quality of the door should be considered in its ability to slow water, and in the door's reliability. Here's how I'd do it:
Quality | Rank | Slow percent | Maximum flow | Burst pressure | Burst/break chance | Burst/disassembled chance | Burst/opened chance |
Base quality door | 0 | 70% | 6 Z-levels | 10 Z-levels ± 5 | 30% | 60% | 10% |
Well-crafted door | 1 | 75% | 5 Z-levels | 20 Z-levels ± 10 | 29.2% | 58.3% | 12.5% |
Finely-crafted door | 2 | 80% | 4 Z-levels | 50 Z-levels ± 25 | 26.7% | 53.3% | 20% |
Superior quality door | 3 | 85% | 3 Z-levels | 100 Z-levels ± 50 | 22.5% | 45% | 32.5% |
Exceptional door | 4 | 90% | 2 Z-levels | 170 Z-levels ± 85 | 16.7% | 33.3% | 50% |
Masterful door | 5 | 95% | 1 Z-level | 260 Z-levels ± 130 | 9.2% | 18.3% | 72.5% |
Artifact door | 6 | 100% | 0 Z-levels | 370 Z-levels ± 185 | 0% | 0% | 100% |
Slow percent determines how much the door will slow down the water, compared to having no door. Slow percent is calculated as 70 + rank*5. For example, if you hit a finely-crafted door with 40 Z-levels of water pressure, it will reduce that to the equivalent of 8 Z-levels of water pressure.
Maximum flow determines how fast water can get through the door assuming you have an infinite amount of water pressure. It's calculated as the equivalent pressure of (6-rank) Z-levels. This doesn't matter for base quality doors - they will always burst before they reach maximum flow. However, if you put 40 Z-levels of water against a finely-crafted door, first, it will reduce the flow by 80%, making it equivalent to just 8 Z-levels. Second, the maximum flow of a finely-crafted door is 4 Z-levels, so we'll reduce 8 to just 4.
Burst pressure determines how many Z-levels of water pressure the door will take before it bursts open. The pressure difference is calculated by finding the maximum difference in pressure between any two passable tiles that are adjacent to and separated by the door. The maximum pressure a door can withstand before it bursts is calculated as 10+rank
2*10.
Burst pressure will not be the same for every door of the same quality. Each door will have its own special burst pressure, and you won't know exactly what it is until you try it. Basically, whatever the burst pressure is you can randomly add or subtract up to one half of that to get the final burst pressure. For example, a base-quality door may burst under as little as 5 Z-levels of water pressure, or it may require as much as 15 Z-levels of water pressure to burst it.
Bursting can have one of three consequences - the door may be opened, disassembled, or broken. If the door is simply opened, it can be closed again (if you're using a lever) when the pressure difference is at least 1/2 of the door's maximum burst pressure. If the door is disassembled, it's been knocked off its hinges and you'll need to have a dwarf repair it. If the door has been broken, there will be nothing left of it.
Burst/break chance is the chance that, when the door bursts, it will get broken. Burst/break chance is calculated as (1-(rank/6)
2) * 30.
Burst/disassembled chance is the chance that, when the door bursts, it will be disassembled. Burst/disassembled chance is calculated as (1-(rank/6)
2) * 60.
Burst/opened chance is the chance that, when the door bursts, it will just be left open. Burst/opened chance is the remainder, so it's calculated as 100% - (Burst/disassembled chance + Burst/break chance).
This will make it so that doors can slow down water, but they're unreliable if you give them too much pressure. If you want to install drowning traps throughout your fortress, you'll either need to rely heavily on high-quality doors, or you'll need to carefully regulate the pressure using floodgates.