I have a custom action that I want to be disabled if the user does not have the ability to edit the item. However, I am using a custom permission level, not Contribute. The permission level is essentially the same as Contribute but with Adds and Deletions removed. I won't have a good way of testing this out until next week, but would the EditListItems mask work for this or is it more specific to Contribute?
I was able to find someone to run a test with last night, and we confirmed that the custom permission level only needs to have the "Edit Items" permission in order for custom actions to hide/show based on the "EditListItems" rights mask.