Welcome... When you say it "fluctuates", is this within a few seconds or minutes..? If this is happens [at steady idle] within a few seconds there may be an issue with the themostat. If it happens every few minutes it is normal themostat cycling.
I have an original radiator and there's a fill mark below the filler neck. If I fill to that mark it runs & cools fine. Even if there's extra coolant, the system will push out the excess and self correct the level. Since I live in the South-West I installed an over-flow tank just to have a little extra coolant.
