The HT 25 probe features a x10000 probe, so one volt displayed equals 10kV. This is the factor applied within the formula to create a custom probe for it.
Not sure about attenuation factor on the cop probe. Generally you don't expect this type of probes to give reliable, absolute numbers. You would use them to get an overall image that will allow you to evaluate shape and time within the obtained waveform and compare coils-secondary events relatively one to another.
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