How do you handle jumping? Say, someone wants to jump a narrow chasm. How do you determine whether they can jump it or not?

combat movement - 10’ with a running start. Otherwise not very far but I haven’t had a player test it yet.
Where is the excitement you might ask? Well I don’t tell the player exactly how long it is unless they have some time to judge to gap. If they barely miss they get a saving throw versus death if there is any chance they could hang on to the ledge or some roots or something.

As a rule of thumb, I’d say a human can:
-from a running start, jump 1.5x (Dex - encumbrance)
-from a standing start, jump .5x (Dex-encumbrance)
-leap up 4’ +/- Dex modifier
The modern olympic record is ~ 27’ for the long jump and 7’ for the high jump. In high school on track team I was a long jumper and could clear about 17’ feet (suggesting I have a DEX of 12).

Alex, what is your reasoning for basing this off Dex instead of Str (which encumbrance is based off)?

Just preference, I suppose. In my mind I tend to visual things related to gross physical movement, like dodging, jumping, and so on, as Dexterity based.
There’s a strong argument that Strength should include the dextrous application of physical force, since it provides to hit adjustments, so then you could have jumping based on Strength.
Or you could be generous and use which ever bonus is higher.