A quick guide help for those who are after this trophy and stuck just short in the 90% + area.
I found a lot of misleading threads and was stuck for ages, But just want to clear a few things up that other threads didn't work on you tube Etc..
I used red rocket and the 6 settlers for starters, which I sent there from other settlements and set up no recruitment beacon.
Got just 1 each of level 3 Store, Clinic, Bar, Food/Drink and Clothing. (Tube videos states you needed something like 14 clinics which you don't as they didn't follow the important note I mention below about arrow capping out first)
A few water pumps, Food , 7 seven beds and defence.
Assigned settlers to stores and 2 to the food.
No NPC's or bought dog(Gene) at the settlement, Only the 6 sent.
Happiness arrow pointing up and done the 24 sleep option(over and over) until happiness arrow no longer appeared next to smiling face and reached 95%. IMPORTANT NOTE: YOU MUST LEAVE ARROW TO CAP OUT BEFORE DOING MORE CHANGES TO SETTLEMENT AND MAKE SURE YOU EXIT AND RE-ENTER BUILD MODE FOR ARROW TO MOVE AGAIN.
So for you guys who are stuck around there my solution was simply to send a settler away back to another settlement(Leaving 5) which raised the arrow again until 99% happiness.
From 99% just sent another away(Leaving 4) and arrow returned and happiness went to 100% in about 1 minute later.
I found that decorations played no part or difference to the arrow after capping out as well as settlement having to be in the yellow stage as mine was green and about half way at Red Rocket. Also found a Synth in my 6 which I replaced with human made no difference for me. Also had food in the workshop stored which I'm unsure it did anything as forgot to check the arrow at the time when storing.
Hopefully this will help others following after getting a lot of miss leading tubes going around the place.