The RMD is the amount you must take for the 2018 tax year... you cannot prorate it.
The 2018 amount must be taken in full by 4/1/19 ... you can take some in 2018 and the balance in 2019 but the entire amount must be withdrawn from the IRA account. You don't have to wait until July to start taking the distributions and if you do wait to take the 2018 distributions in 2019 remember that the 2019 distribution MUST be made in 2019. The option to wait is ONLY allowed for the first RMD.
