Wearing a vest
Most people who try out vests, not for work, tend to have them a bit low, so put your finger at the notch, at the base of the throat, then about an inch below that, treat that as your aiming spot, actually from that point, and up.
So, 10 or 12 inches vertical, 2.5 " wide bar, throw rounds at it!
Use a IPSC/USPSA cardboard target for practice, pen lines from eye level to 12" down, 2.5" wide. Buy the Target Barn, 3/4" wide patch tape, way better than the wee square patches, patch every two magazines.
Distances? 6' to 15'? What ever. Bursts of 3 or 4? up to you.
Both hands/single/ and not forgetting weak hand. Do this in a panic, from the holster, while taking a stride, left and right. One hundred rounds goes fast!
Keep Safe.