I wish i could say I found the answer, but alas someone else figured it out and i thought I would pass it on.
Simply put, the answer to the life, the universe, and everything, is a Hot Shower.
42 celcius is ~108 Farhenheit. If you want a hot shower, you set the water heater/boiler temperature near this number. Douglas Adams lived in England, so it makes sense he would use celsius.
Think about all the connections in the book to towels....yet never for one of their main purposes, drying off.
If you were a hitchiker stuck in a vogon cargo hold....wouldn't the first thing you want when you get to the Heart of Gold, is a hot shower? I mean you have your towel ready, right? But no, Arthur perpetually in his robe, never showering, always stumbling around. All he needs is a hot shower! Ideally with Trillian!
Where do people do some of their best thinking?
I'm sure there are more connections but I have not read the book again since.
Thank you Douglas Adams, you are a true genius.
Your secret lasted a long time...but its time to tell the world.