Warning: Guy rambling here, migh not reach conclusions.
The amount of grief generated at any given time depends on the amount of grief in the soul gem.
The capacity of controlling said grief also depends on the amount of grief in the sg, but the relationship is inverse.
On the potential, on a grief spiral the amount of grief increases until the sg is filled, at that point peak grief is reached and the sg beomes a grief seed.
If the grief is extracted with the purpose of leaving the sg with a given amount, the grief output depends directly on the amount present in the sg. The closer to the breaking point, the higher the output.
On the danger side, on a grief spiral our capacity to control becomes zero at the breaking point. It may be an abrupt change, one moment we are able to extract grief, the other we are not.
So the question is, what's the highest amount of grief that can be maintained in the soul gem? So far, we know that maintaining the sg at 0% is possible, but we should test at 20%, 50% and in company, at 70%
If successful the technique would allow to create grief before battle, and we wouln't need to carry grief all the time.