Di tanks normally come with 3/4 fittings on. So look on ebay or other sites for a 3/4 to 1/4 push fit fitting. That will allow your pipe from ro to fit snug into the di vessel. You can even keep the outlet as a 3/4 and connect a normal hosepipe connection to it as that simply feeds into your ibc tank.
So i started to fill my virtually empty ibc tank yesterday 7pm, im about 200 -300 ltr short of it being full. So prob by 10pm. So about 27 or so hrs to fill it.
Not sure if this angle helps better,