When Marriott combined programs on August 18th they took all Starwood points balances and tripled them. And they kept the basics of the Starwood points transfers to miles system. So miles transfer 3:1 with most airline partners (instead of the old 1:1, but it’s effectively the same) and transfers into 20,000 miles earns 5000 bonus miles.
In other words, transferring 60,000 Marriott points yields 25,000 airline miles. When you transfer to United you also get a 10% bonus because of the Marriott-United RewardsPlus tie-in. And right now United is offering a 30% bonus on Marriott transfers up to 25,000 extra miles.