Minecraft Client Coding Drawrect triangle

[tari...@gmail.com]

Hey i have following problem 

i want that at the menu are a light transparent triangle is 

i made this with     

Gui.drawRect(375,178, 585, 292, 0x90000000);

in fullscreen its right there where i want it but when i go in windowedmode or make the window smaller its not there where i want it so how can i make that the triangle is there where i want i every windowmode ?

thank you 

sry for my english im from germany