ZDoom uses a special coordinate system for HudMessages. In 4:3 resolutions, the coordinate system's rectangle matches that of the screen. In aspect ratios other than that, a 4:3 rectangle is put at the center of the screen and the remaining screen space is out of its boundaries.
The HUD coordinate system's rectangle width and height are specified in SetHudSize.
ACSUtils provides functions to correctly work with the HUD coordinate system.
GetAspectRatio returns the aspect ratio that ZDoom assumes the screen is. ZDoom uses tricky code for that.
HudLeftFor, HudRightFor, HudTopFor and HudBottomFor return actual screen boundaries in the HUD coordinate system for the specified width and height of the HUD coordinate system, taking the current aspect ratio into account.
Put a message in the top left corner of the screen:
HudSetVirtualSize(640.0, 480.0); x = HudLeft(); // This will be negative in widescreen. y = 0.0;