Hi rockoyster and Bryan & Susan,
>> If you are using the pass on a day for which you have not written the date on the pass is it just a matter of rolling up to a ticket office to discover how much discount you'll get or is there a reliable way of predicting beforehand what discount you'll get on what trips.
All travel benefits are predictable (but can change from year to year), and detailed for each country for the current year on the Eurail site. For Switzerland, details are here:
>> What I don't get is whether the map on page 3 of the PDF you referred to applies only to "travel days" or any day within the Flex validity period.
Both, because that map shows both free and discounted travel routes. Free travel uses a travel day, but discounted travel doesn't.
From the 2017 Eurail Pass Guide supplied with every Eurail Pass and also available online:
For Flexi Pass holders: Discounted travel and other benefits do not require use of a travel day. You can make use of the benefits at any time (even if you have used all your travel days) as long as they are used on a day that falls within your overall Pass validity.
By the way, the map on page 3 that you referred to is a separate map produced by the SBB, which has been attached to the map produced by Eurail. For Switzerland, I much prefer the SBB map, as it makes it much easier to compare what is covered by the Swiss Travel Pass with what is covered by the Eurail Pass.
The SBB has all its network and validity maps available for download from the "Downloads" area on the lower right-hand side of this page:
I hope that makes things clearer.