I think you are best off making the payment now. Basically they look at your payments on a cumulative basis. So if they think by 6-15 you should have paid $2,000 and you only paid a $1,000 they will charge you interest on the difference until it is paid. Usually not a lot of money but IMHO they don’t spend it well so why give them more than we havet to? We can use another trick to get rid of interest if you don’t convert to a roth until after 9-15.