Jabird
Your first port of call should be to visit the HMRC website and check out the rules. You are correct in that the total length of your journey is liable for APD but there are certain exemptions a stay of more than 24 hours in say CDG is normally enough to break the journey, but read the rules until you fall a sleep!
Also the GC distance is based on the capital of the country you are visiting so for example Hawaii is part of the US and is classed the same as Washington and you would pay less tax than flying to the Caribbean !
From Dublin the tax is only 3 euros even if you fly to AMS then long haul back west! But you have to factoring getting to Dublin and even FR go tech from time to time, of course you can avoid APD altogether if you fly by a private jet!
I think they even get you if you fly in economy to CDG & business the rest of the journey they still charge full business APD for the part of the journey they have no tax right over
Good luck