This is a reasonably complex Estimated Position in that we are in the Southern Peninsula on SPDST, and with tidal stream information referred to HW Victoria in the Northern Territories we are working across two time zones. We set off and then have to alter course to avoid a tug and her tow and then resume our course. We are working across two tidal hours, we have to use the computation of rates table and of course we are dealing with deviation. All perfectly manageable if you break it down into bite sized chunks.