Using a one row filtered portal with a summary field in it works to display the total, but the total displayed is not accessible to scripts nor calculations.
But you can define a relationship for the first portal that does not require a portal filter and that, in turn, is a relationship that you can use in your calculation.
You'll need to substitute your names for mine, but this relationship will match to your selected records
[Put any pairs of match fields defined for your current portal here ] AND
LayoutTable::constY = PortalTable::Select
Define constY as a calculation field with just "Y"--the value in your select field as its sole term.
If you have FileMaker 12, there also are ways to get these totals using ExecuteSQL.
Replying to my own post here. After two weeks of struggle with this, I think the light just went on while I was re-reading my own post. I added a "SelectedForPrint" field to my parent table with Auto Enter "Y" and made that part of the relationship with the child (portal). DUH! Appears to be working.
Is this the best solution? I can remove the filters?
Our posts "crossed in the mail". Thanks for the confirmation. Should have checked in sooner.
Note that if you have two users clicking "select" at the same time, all the selected portal records will appear for both users.