Hi,
I wish i could remember more about how we did this back in the 1980's, but going by the way a magnetic core responds i would say that the voltage goes down with the frequency, so that if the frequency is reduced by a factor A then the voltage must also be reduced by a factor A. So for example if the normal run frequency was 50Hz and the normal voltage was 100 volts, then reducing the frequency to 40 Hz means you should reduce the voltage to 80 volts.
I would not be surprised to see the slow start keep the voltage lower than that requirement for some time also, as it gradually ramps up to meet the frequency factor. So if the frequency of that same motor was 20 Hz and the normal voltage for that frequency was 40 volts, then perhaps starting at 0 volts and ramping up to 40 volts would work well, then start changing the frequency too if needed.