Encumbrance (yes, again)

After a lot of thought, my conclusion on encumbrance, until I inevitably revisit the topic, is this:

You can carry ten "things".

(Note that ten is chosen as a nice round number. It is deliberately a tight limit, and it deliberately does not factor in the character's strength, race, or other factors. It's chosen to be easy, not realistic.)

A "thing" is anything you consider important enough to declare that your character definitely has: your armour, weapons, spellbook, whatever. If you want to be sure your character has rope, you need to list it as one of your "things". If you want to be sure your character has a red robe, you need to list it as one of your "things". If you also want to be sure your character has a blue robe, that would be another "thing".

A reasonable number of identical items can be considered a single "thing". So if your character carries 3 javelins, or 6 throwing knives, or similar, that would be a "thing". Where you carry a weapon that uses ammunition, that ammunition is included (but only mundane ammunition - if you also carry various special types of arrow, you'll need to list those separately).

You don't need to list containers like your backpack, or whatever, amongst your "things". However, neither can you list "a backpack containing {ten various items}" as a "thing" - feel free to specify some or all of those ten various items amongst your "things", but they will count separately.

Finally, any magic items you want to carry must be listed amongst your "things". These are important enough to the game that if you don't specify that you're carrying them, you're not carrying them.

It must be noted that your ten "things" are only the items you've specified that your character is definitely carrying. Seasoned adventurers, such as the PCs, probably carry a bunch of other equipment.

So, what happens if the party needs something that nobody has listed as a "thing"?

Firstly, the DM should make a ruling: is this something a seasoned adventurer would obviously have with them, something they probably would have with them, something they might have with them, or something they wouldn't have with them? (Also, are they in an environment where they could easily rig up a substitute? If so, improve the odds one step.)

In the first and last cases, the answer is obvious: the party either does or does not have the item. For instance, when taking a week's journey from A to B, of course the party would pack rations for that week (and a bit extra). Conversely, they wouldn't pack a key for a lock somewhere in the depths of the dungeon (since they have no way of knowing they'll encounter it).

In the case where they would probably have the item, there's a flat 70% chance that one of the PCs has the item. If they might have the items, it's a 50% chance. Roll the dice, and find out! (Note that this is a chance for the group as a whole, not a roll for each PC individually!)

To be strictly correct, the DM should probably maintain a list of previous rolls, noting which events are and are not present in the group. After all, if you didn't have rope five minutes ago, you won't have it now! That list should reset when the party takes downtime in a settlement. (Note that this may mean they set out again leaving behind the rope that they had previously. That's a feature, not a bug: if the party wanted to make sure they had it, someone should list it as a "thing".)

One last detail: if the PCs have checked for and determine that they have some item (let's say rope) and then, for whatever reason, they use up or lose that rope. In this case, the DM should immediately roll again to see if they have another item of that type. Here, the chance is 33% - it's much less likely that they'd carry two coils of rope than that they'd carry one. This same percentage chance also applies if a PC has specified an item as one of his "things" and then used it up.

So, to recap:

Would a seasoned adventurer be sure to pack this item?
Obviously: 100% chance
Probably: 70% chance
Maybe: 50% chance
No: 0% chance
Duplicate item: 33% chance