Proximity effect and losses in an SMPS Flyback transformer?

Hello,
The following is a 150W offline Flyback ( abbreviated schem attached) with an output of 300V.
The three secondaries would each be spread across their own layer layer in the transformer. The three secondaries would be interleaved between the two primary halves to reduce leakage inductance.
-So, Given that the transformer has five layers of coils, to what degree do you believe that “Proximity Loss” would reduce the efficiency?

(it’s a constant off time flyback and switching frequency is around, but not exactly 90kHz.)

Until you actually build a transformer there is no way of knowing.
There are too many variables in building a transformer. example " The three secondaries would each be spread across their own layer layer in the transformer." There are many different ways of doing that.

Flyback said:

two primary halves to reduce leakage inductance.

Click to expand...

p-------
s-------
s-------
s-------
p-------
Think about the p to s capacitance.
p------
p------
Now think about the p to p capacitance. Hard to know but probably there is not a great difference.
In example 1 there are many layers of insulation between P-S while in example 2 there could be less layers so more capacitance.