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Your derived field is correct in logic… but it’s using the wrong scope.
That’s why you’re getting a huge amount of “No value.”

Session starts is visit-scoped,
but your derived field is being evaluated at the hit level, not the visit level.
So only the first hit in each visit gets the Page Name — every other hit becomes “No value.”

That’s exactly what you’re seeing.

Change the derived field scope to "Session"

Edit your derived field → in the top right, set Scope = Session (not Hit).

When you do this the following happens:

• When the session starts → CJA picks the first Page Name

• That value is then carried for the entire session

• No more “No value,” because the dimension is no longer evaluated on every hit

If you want it even simpler:

Use Entry Page (built-in)
In many CJA connections, Entry Page already exists.
Search for it first — it works perfectly and avoids needing a custom rule.

The reason why your rule created "No value" is because:

• You only set the value on the first hit

• Every other hit in the visit has no Page Name set → so your “Otherwise = No value” takes over

• When you break down Sessions by the derived field, most visits show “No value” because you’re accidentally counting every non-first hit

Session scope fixes that immediately.