Well in response to your post.
As they say I wouldn't be asking if I had the pieces to the jigsaw

The facts I have are as follows, Robert Johnston was born on 26 August 1868 at 16 Luton Street, Kirkdale, Liverpool.
His parents were Robert Johnston an Engine Fitter/ Commercial Traveller/ Seagoing Engineer born 1833 and his wife Mary born 1845 in Greenock. Their eldest daughter was Elizabeth also referred to as Isabelle born in 1863 in Glasgow.
However, the information I have provided will be of no use to what I am going to describe. I have in my possession a piece of paper written in the 1980's.
As written:
Fathers Family. Captain and Lady Booth from the Booth Shipping Line They had one daughter she married the engineer off her fathers ship her name was Margaret Edwina Booth and his name was Robert Johnston I they had four children, your late father Robert Johnston II . Having attempted to research both the Albert Booth and Company and the Booth Steamship Company and their family tree. I cannot even find a Margaret Edwina Booth.
On Robert born 1868, on his baptism it states mother Mary not a Margaret. As well as the subsequent census. Therefore I assume this is just a family tale passed down over years with no concrete evidence or proof.