Team,
Try this:
In cell A1, add following "custom" validation:
=not(isblank(b1))

As I see it, this should restrict leaving cell A1 unless cell B1 already has
a value.

I've also tried =b1<"" and some others. Should be simple but I can't get
it to work. Validation will not fire. For each equation I've tried, I've
cut and pasted the same equation into a field and it works fine.

Thanks.