At the top of this thread, on the right hand side of the page in blue letters is "Watch Thread" . Click on that and follow the popup.
Then, tomorrow when you come back, click on "Watched Threads", which is on the bar directly beneath the Websleuths header.
From there you can "Go to first unread", which takes you to your first unread message.
Or there is another way to find where you left off. At the beginning of each thread in your watched list, is a tiny green-blue dot. If you click on the dot, it will take you right to the place where you left off reading.
If this doesn't make sense, perhaps a techno savy person could provide you with screen shots.
BTW, thank you for not saying "marking my spot". That always conjures up an image of a dog peeing on the thread. :/ jmo