OPT equalising circuit

Hello.

I noticed that in most cases a capacitor or a RC circuit is connected
across the primary of a tube OPT. It is apparently to compensate for
the leakage inductance of the transformer.

However, I am not sure how to correctly calculate this RC circuit.

If I know my load impedance Rl, if I know output tube internal
resistance Ra, if I know (measured) the actual leakage inductance Ls
(with the secondary short), if I know the highest frequency (Fmax) my
amplifier is intended to reproduce (15kHz), then what is the correct
way to set this RC circuit?

Who could help or refer to any article?

Is it simply R=Rl and Xc shall be so that Xc(Fmax)=XLs(Fmax)=Rl?
Or is it more tricky?

Regards,
Alex