No idea what the calculation gibberish is there for, but if your dexterity is 18 and the number rolled on 1D6 is 4 then 8 is the correct damage.
Hi Dwig, no need to understand if it's too hard for you. Just open a browser on google and paste the operation from the screenshot showing the damage:
4 * ((4 - 1)/2 + 1)
See the answer it returns, provide your observation. good luck.