Hi Dverna,
I can be totally wrong and apologize for any miscommunication. The distance is a guess based on a 50 yard target on the range as well. The pistol targets seem to be half of that.
Attached is a photo of the targets in the background. I know it's more than 18" based on the length of my father's 18' boat. Would my father's 21' boat reach the targets. Not quite.
I use an Amazon shipping bag filled with the dirt and bullets from this berm, bench shoot my handgun from this, and have a folding stool to shoot from.