Jundy,
You have probably already read that to answer your question you cannot simply apply a simple rule of three calculation to resize a toroidal core for a specific criterion.
As Owen Duffy has repeatedly shown for years, the efficiency of an impedance transformer is determined by 2 main criteria
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the quotient of the toroid cross-section and the mean field line length and
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the magnetizing impedance created by the primary winding.
The calculated transformer efficiency for 2 stacked FT-140-43 cores with 3 primary turns is approx. 92%.
For operating modes such as SSB (and CW) 2 stacked FT-140-43 ferrite cores could just about suffice for a transmission power of up to 100 watts, but only if 3 primary turns are used.
Enamelled copper wire of 0.7 or 0.75 mm and a mica capacitor 100 pF/500 volts are recommended, as well as tight winding without gaps and short connecting wires from the transformer and capacitor.
Remarks
Impedance transformers with high coupling ratios in the range of 1:49 to 81 are suitable for use with half wave antennas or multiples thereof. If the wire length is precisely adjusted for a specific antenna geometry, it is possible to use several (harmonic) frequency bands without an antenna tuner.
When operating with random wire lengths, not only an antenna tuner is mandatory, the common mode currents that occur with high SWR could make the use of a common mode choke necessary (e.g. in the coaxial cable about 3m after the impedance transformer).
Many of these serious problems do not interfere with QRP transmit power and can therefore be neglected. If you know what I mean, hi.
73 gl, Heinz