If the B+ is fed at the bottom end of the winding, the plate of the modulator tube at the lowest tap and the final at the highest tap, that will allow 100% modulation. Otherwise, with just a choke with no taps, the modulation will be limited to around 75-80%, unless a dropping resistor by-passed with a capacitor is inserted in the B+ line to the final. The extra higher taps will allow 100% modulation, but there will be no DC magnetic field cancellation. The mid-tapped choke or transformer winding can be used with a push-pull modulator and the +HV fed to the mid-tap, but that presents a 2:1 step-down turns ratio, which will not allow 100% modulation. "Modulation autotransformers" are designed to have some extra turns added to one end to present a normal modulation transformer step-down ratio. Several commercial rigs used those. Timtron says he has had good results by "turbo-connecting" the power transformer, so that the mid-tapped HV winding is used with the push-pull modulator, and the mains voltage primary winding is added in series at one end of the mid-tapped winding to give less step-down. Whether or not this works depends on the overall turns ratio, and whether or not the primary winding is insulated well enough to withstand the full modulated B+ voltage. This may work OK at the 100-watt class, but I think would be risky at higher power with the B+ in the thousands of volts. Plus, the amount of step-up achievable with the primary would likely not be enough for 100% modulation capability.