TMZ has learned that they have formally agreed Kevin will gets custody of their 2 boys 70% of the time. We're told this has actually been the arrangement since August of last year but wanted to see how it worked before formalizing it -- and they made it official last week in docs filed on Aug. 28.

This is a shift from the previous agreement where they each had 50-50 custody. It's been that way ever since Britney stabilized following her mental health struggles that began in 2007.

Sources with knowledge tell us under the new arrangement Britney's 30% custodial rights with 12-year-old Jayden and 13-year-old Sean are not supervised.

TMZ has been reporting ... Britney has struggled over the past year with her mental health, in no small part because the medicines she was taking for her mental health condition stopped working. She entered a facility earlier this year to stabilize herself ... however, our sources say doctors were having trouble finding the right mix of medications.

Britney's been out and about in public, but privately ... we're told she still struggles to get back on an even keel. Her career remains on ice while she tries to get better.