I've always used the value of 6mm as a maximum value of the gap between a fixed nip guard and a cylinder or roller. This is the figure given in HSE guides such as Printing Information Sheet 1 and makes sense as fingers are larger than this.
However, I've just been made aware of an accident where an operator on a laminating machine had his little finger drawn into the gap shown below. We don't know the precise gap as the nip guard was removed to release his finger, but the slack in the fixing holes allow the gap to be between 1mm and 5mm. Although the roller is made of very stiff rubber, it deformed so that his finger became trapped.
It may be that this is a freak accident as there are thousands of such immediate guards in place and I am not aware of any similar accidents.
However, it makes sense for all those who have machinery with such guards to adjust them so that the gap is as small as possible.