In the interest of improving my effect for future versions: do I understand that the problem here is that you'd want (need) the set the blur radius to something between 75 and 100?
Also, if you want the shadow to be darker, one way to get that is to use widening with bluring. As long as widening radius is less than blur radius, you will not get "unblurred" full opacity pixels in your shadow. You can think of the widening happening before the blur. And you can think of the widening as being (more or less) the same as an object outline. But of course, the blur only blurs the shadow, not the original image.
The "keep original image" checkbox is present, precisely to allow this kind of scenario. But if you turn it off, the effect becomes significantly slower. If I understand your needs correctly (i.e. a blur radius of 100 pixels), I'm thinking you'd get your result faster with such a blur range in my effect and "keep original image" on, than having to do these multiple steps with duplicating the layer and separate blur. If that's true, I could increase the ranges in the next version.