Default [Q] flashing rom after going back to stock

Hello ! I was rooted with poweruser rom and decided to go back to stock i used the reset flashtool and successfully got back to stock but had a problem with the flashtool "Factory Reset Flag = 6
Factory Reset Flag Check Fail!!!
00000064
Error: Factory Reset Flag Check Failed" but i got to the homescreen(lg user interface) so i didnt really pay much attention to it. I want to root the phone and flash poweruser rom but had a doubt though, will flashing delete everything on the phone including stock image ,absolutely everything ? Please say yes ! I want to only be running the poweruser rom and not the stock image too from resetting the phone. Please help me !!