Currently P4_CHANGELIST gives the last sync changeset id for a depot. On depots which containing a single project this will match the last changeset for that project, but when a depot hosts multiple projects the last sync'd changeset can be greater than the last changeset that affected an individual project.

I've tested a quick patch that creates an environment variable P4_LAST_CHANGELIST with the last changeset id related to the current project, which is available here;

https://github.com/namtabmai/perforce-plugin/commit/765dff821abf46a5eace30c28b2a991f294e38fe

I'm not overly happy with that env name but it at least seems to work as expected on the builds I've tested it on.