I do not understand - perhaps a better example?

CB does not have such a field. Couldn't the user just provide the math themselves and update the field to 20000? Otherwise you'd need to develop a new fieldtype plugin in order to provide such a field. You could use ajax file fieldtype plugin as an example.

Ok, this can be done.. will be tricky, but can be done. You'll need 2 fields, both being Integer types. Your first field would be for example cb_adddistance then your second would be for example cb_distance. The cb_distance field needs to be Read Only. You will need CB Fields incubator project for this to work. Once installed configure 2 triggers. The first trigger will be for example the below setup.

(this one must be first! change the order if necessary)
Trigger: onAfterUserUpdate,onAfterUpdateUser
Access: Registered (add more usergroups if you like)
Conditional:
-Field: cb_adddistance
-Operator: Not Empty
Field: cb_distance
Operator: Add
Value: [cb_adddistance]

The above will take cb_adddistance and add it to the read only field cb_distance. Now that we've updated the distance we need to empty out cb_adddistance. To do this you'll need a second trigger configured as follows.

(this one must be second! change the order if necessary)
Trigger: onAfterUserUpdate,onAfterUpdateUser
Access: Registered (add more usergroups if you like)
Field: cb_adddistance
Operator: Set
Value: (leave blank)

With the above it will empty the cb_adddistancefield so user can add more distance as they see fit.