To give you an idea, here's my (incomplete) planning doc from the
Mersey 24 in 2013.
Here's my instructions to my helper for the
RTTC 12 in 2015.
The Mersey is a simpler course than the ESCA: until you go to the finishing circuit, it's up and down the main leg plus some laps of the smaller Quina Brook circuit, all of which pass Prees.
24 hours is a long time for Mr S to be a lone helper, so you may want to find him a friend. I had two helpers on the Mersey 2013 and based them at Prees almost the whole time: we did one hand-up at Espley on the first leg south, then they stuck at Prees until it was time to go to the finishing circuit. With two of them, I could take on a fair amount of supplies so this worked.
On the 12 I had one helper, who based herself at Prees for the QB laps and Espley for the main laps. This also worked.
If Mr S is based at Prees, get him to hand up before the roundabout. Then if you miss the catch, he can run across the road and hand up on the other side.
[ETA one more: Do a dress rehearsal. Before this year's 12 I rode the first ~120 miles of my schedule at my planned heart rate, on the local 12 hour course, in full kit, taking food and drink when I planned to do it in the real thing. That showed me whether my schedule was realistic and whether I needed to alter my setup.]