You are correct in the first part, use the difference in HW Dover to find the correct tidal stream chart to use.
The tidal stream charts
in Reeds are taken from the Admiralty Tidal Stream Atlas's.
These also use HW Dover as the base point.
But, they also show the time difference from HW Dover to HW at the most convenient local standard port for the area of the tidal stream atlas.
I'm looking at NP 253 Tidal Stream Atlas North Sea Eastern Part.
It shows the HW difference between Dover and Helgoland (maybe Helgoland is Germany's equivalent of Dover)
So, you could have Tidal Stream Atlas, but not the times for HW Dover, but you may have the HW times for Helgoland (because it might be assumed you have the tide data for the area you are in)
So, knowing the time difference, you can find the correct page.
In you case, HW at Dielette is 4h 30m before HW Dover, so the Chartlet which is labeled HIGH WATER
DOVER, can also be labeled as 4h 30m before HW Dielette.
Only really useful if you dont have HW times for Dover, but if you have Reeds, you should have those times, but who knows, you might have only taken Area 18 with you for the trip.