custom field which helps in doing add/subtract operations from other custom field

[Randhir Kumar]

This question is in reference to Atlassian Documentation: Adding a custom field

custom field which helps in doing add/subtract operations from other custom field

[Nic Brough -Adaptavist-]

You'll need to write a custom field type add-on to do that, but I'd strongly recommend using one of the addons in the marketplace that can do it.  Script Runner can do scripted fields, and it's the only one I'm familiar with.  You may find others suit you better though.