try it with brackets
var panel_pos_x=(screen_size.x/2)-50;
var panel_pos_y=(screen_size.y/2)-25;
perhaps it will work
Hope that helps
here is something out of the manual with centered Panel (:

// startup with 300x300 window resolution
function main()
{
screen_size.x = 300; screen_size.y = 300;
...}

// place panel in the center of the screen
function center_panel(PANEL* panel)
{
panel.pos_x = (screen_size.x - bmap_width(panel.bmap))/2;
panel.pos_y = (screen_size.y - bmap_height(panel.bmap))/2;
}

WFG Progger