Offline Flyback: Inner layer creepage/clearance?

[Flyback]

Hi,
May i please ask for any good google search terms for finding out what is the minimum allowable clearance/creepage between inner layer copper on primary and secondary of an offline SMPS. (across the isolation barrier)
For outer layer copper its 6mm, but much harder to find out the figure for inner layer copper?

 

[Tony Stewart]

Inner layer copper with Kapton tape 1mil 25 um is 300 kV/mm or 7.5 kV

 

[ronsimpson]

I got some transformer tape in 3mm wide roles. (maybe it was 2mm I don't remember) I built up spacers to keep the coper away from the walls in the transformer. Much like the pictures here. Then the Primary had to have sleeves on the wire as it entered the winding.

Later I moved to triple insulated wire and it was much better. No creepage problems, wind to the edge of the bobbin is ok.

 

[Flyback]

Thanks, and your answers are of superb interest in there own right.
Though the question of the top post still stands if i may so be permitted?
I do not ask for the answer, this one is one of the most illusive questions in the world of offline SMPS....just ideas on where might be a good place to search for the info?.....IPC2221A does not have the info...neither does EN62368.
Nor scores of other places.