No set value of weight.
When you are in blendshapePaintWeightTool it's possible to choose to paint enveloppe or target enveloppe.
I want to select a specific target enveloppe when I launch this tool.
I have tool for edit, extract, substract, etc blendshape and it's really good feature for my user when he launch my "targetEditor" the target enveloppe item is automatically selected.