The level is best WHERE IT WANTS TO BE, I always fill to about 1/2" to 1" above the minimum mark, as this is plenty, you know it has coolant, the purpose of the tank is to allow/provide an air space to allow expansion, hence it's name.
If it's got slightly more in than it wants, it will settle at it's level, the mondy I had never used a drop of water in 3 years, but level even when topped to minimum always settled just about 1/2" below minimum, never budged from there ever apart from of course when hot, I'd top up few days later that same spot again, so left it and there it stayed.
You may be lucky and not have killed the headgasket only time will tell.
tabetha