I think it's better to have the wounds based on the PC's maximum hit points rather than a straight number for scaling reasons. 10 hit points means something entirely different to a level 1 PC as it does to a level 20.
True, it does.
But at 20th level, PCs have access to a LOT of resources that they did not have a 1st level. Damage Resistance. Higher CON. Magic Items.
At 20th level, PCs are fighting Dragons the sizes of small buildings. But, PCs are still human (or elven or whatever).
Doesn't it make sense that a Dragon the size of a small building should hit the Fighter at 20th level and crack some ribs on solid hits more often than the Kobold did at first level?
The difference is that this Fighter has some Damage Resistant armor. This Fighter has Potions in his pouch for just such an occasion. This Fighter just needs to survive the fight and his team can brew some herbs. This Fighter has a boon from the High Priest in the city. This Fighter had a CON boost cast on him and is healthier (i.e. more CON) than he was at 1st level.
The first level Fighter doesn't have any of these "Go to the Well" abilities. He might have a Healer in the party.
So, I think that if you scale it with hit points, it stops working because of the vast plethora of abilities that PCs acquire.
If it equates to CON, then certain threats are equally threatening to everyone with the same CON from a sheer wound damage perspective, but higher level PCs can handle the bigger threats more because of their abilities, not just because they are "bags of hit points".
As for adding up or subtracting down, it doesn't matter too much. With normal hit points in various editions of D&D, I've seen players do both (and I've done both myself).