I would do
Lua Code:
local height = GuiRoot:GetHeight()
myFrame:SetAnchor(TOP, GuiRoot, TOP, 0, height/4)
/edit: if you want the center of your frame to be at that point, then do this instead:
Lua Code:
local height = GuiRoot:GetHeight()
myFrame:SetAnchor(CENTER, GuiRoot, TOP, 0, height/4)