Resistance based on map help

[bojangles2872]

I have this line of code that I created

for (int i = 0; i < Resistances.Length; ++i)
            {
                Resistances[i] = 0;
                if (base.Map == Map.Felucca)//bojangles resists by facet
                {
                    Resistances[i] -= 50;
                }
                if (base.Map == Map.Ilshenar)
                {
                    Resistances[i] -= 10;
                }
                if (base.Map == Map.Trammel)
                {
                    Resistances[i] += 10;
                }
            }

But the issue I am having is when I am in Felucca and have Magic Res set to 200 my character doesn't have -20 resistance, instead I have 30 resistance. What I am aiming for is Magic Res to add onto the players current resistance based on what map they are on.
I have also changed my Magic Res algorithm to this.

public override int GetMinResistance(ResistanceType type)
        {
            int magicResist = (int)(Skills[SkillName.MagicResist].Value * 10);
            int min = int.MinValue;

            if (magicResist >= 1000)
            {
                min = 10 + ((magicResist - 1000) / 50);
            }
            else if (magicResist >= 400)
            {
                min = (magicResist - 400) / 15;
            }
            
            return Math.Max(MinPlayerResistance, Math.Min(MaxPlayerResistance, min));
        }