Assume the following: The mid-qrg of the WSPR band on 30m is 10140.2kHz. With a "BFO" setting of 1.5kHz, that gives the famous 10138.7kHz. The subharmonic receiver's LO runs on about 5068.8kHz (depending on the individual canned oscillator). Double that, it will give 10137.6kHz, resulting in a "BFO" frequency of 2.6kHz to reach the WSPR-band.
My resulting LO frequency is a about 69Hz further off, here comes a correction in, one has to measure first. For me, it results in a "Dial Frequency" of 10.137531MHz and a "Rx BFO" of 2669Hz.
Good news for WSPR-users, I will be now monitoring whenever the grabber is set to 30m.