No. It's not correct.
local nScreenX, nScreenY = guiGetScreenSize( )
--[[
nPosX = 0.5
nPosY = 0.5
nSizeX = 0.5
nSizeY = 0.5
]]
nPosX = nPosX * nScreenX
nPosY = nPosY * nScreenY
nSizeX = nSizeX * nScreenX
nSizeY = nSizeY * nScreenY
guiCreateWindow( nPosX, nPosY, nSizeX, nSizeY, '...', false )
Anyway you can use relative argument.
guiCreateWindow( 0.5, 0.5, 0.5, 0.5, '...', true )