Setting Stop loss

Hi Pat, do you use 8 EMA or 8 SMA ? i have it set to 8 EMA but is a such a short MA that premarket is one price then fluatiates every hour forward.I would like to set a hard stop 20c below it premarket and to be sure thats the accurate price but how when it's changing so much throught the day. Do i do it from the close EMA price, the open..? Thank you!