Brad Delong suggests we need a combination of monetary and fiscal policy. I am not opposed to that but I think the statement could do it alone.
You would say something like the following.
The Committee judges that economic conditions will warrant exceptionally low levels for the Federal Funds rate until current value Gross Domestic Product exceeds $19.5 Trillion.
$19.5 Trillion is the level we would have reached in late 2013 had we continued on a 5% growth path. The key is coming up with the target date then you derive the NGDP number and set the funds rate conditional upon that.
I think coming up with something like a Taylor rule for NGDP is useful. If its already be done let me know, if not I am on it.