At this link, I asked a while ago about some help with this thread’s techniques for making arcs of circles in decals.
No one answered for several months, so I thought I’d just post a little reminder linking the question.
first, create a material and set up it for a decal.
now make a gray mask for your decal in photoshop or paint(I used paint 
) or … . you must use black for the part you want to be invisible and white for the other part and if you use between black and white you can achieve that border effect.
now connect that image to Opacity in material and you have what you need. just make sure your decal touches the ground!
now go to your character and go to “Add Component” and select decal and set up transform of decal
now if you want to make your decal bigger just change scale in transform and you have it
Thank you! This was actually one of the first ideas that occurred to me, but the problem is that I just make one image for one aiming thing, and scaling it makes it look stretched and weird. (Plus, any imperfections are very noticeable.) I was hoping for something that used the arc-stuff from the thread I linked, with some after-arc-is-calculated opacity changes. ((I also put this comment on the original))