Hi,
a window is a panel element that shows a part of a bitmap. The variables set the offset of the upper-left corner of the window from the upper-left corner of the bitmap in pixels.

Look at templates bluebar.pcx file. The upper half part of the bitmap is black (translucent in the engine) and the lower half part is the bar itself. This way, the upper half part is shown when the bar is empty, so the offset is 0. And the lower half part is shown when the bar is full, so the offset is the half of the bitmap height.

With this bitmap<->window realationship in mind, you only need to factorize the health by a healthMax (or so) variable and multiply it by the half of the bitmap height.

Code:
#include <acknex.h>
#define PRAGMA_PATH "%EXE_DIR%templatesimages"

FONT *fntCourier20b = "Courier New#20b";
BMAP *bmpBar = "bluebar.pcx";
var nHealthBar = 0;
var nHealthMax = 100;
var nHealth = 100;

PANEL *panHealth = {
	pos_x = 100;
	pos_y = 100;
	window ( 0, 0, 5, 80, bmpBar, 0, nHealthBar );
	digits ( 20, 40, "Health     [Q+][A-]: %3.1f", fntCourier20b, 1, nHealth );
	digits ( 20, 60, "Health Max [W+][S-]: %3.1f", fntCourier20b, 1, nHealthMax );
	flags = SHOW;
}

function healthBarUpdate () {
	nHealthBar = ( nHealth / nHealthMax ) * ( bmap_height(bmpBar) / 2 );
}

function healthUp () {
	while ( key_q ) {
		nHealth = minv ( nHealth + 5 * time_step, nHealthMax );
		healthBarUpdate ();
		wait(1);
	}
}

function healthDown () {
	while ( key_a ) {
		nHealth = maxv ( nHealth - 5 * time_step, 0 );
		healthBarUpdate ();
		wait(1);
	}
}

function healthMaxUp () {
	while ( key_w ) {
		nHealthMax += 5 * time_step;
		healthBarUpdate ();
		wait(1);
	}
}

function healthMaxDown () {
	while ( key_s ) {
		nHealthMax = maxv ( nHealthMax - 5 * time_step, 1 );
		nHealth = minv ( nHealth, nHealthMax ); 
		healthBarUpdate ();
		wait(1);
	}
}

function main () {
	fps_max = 60;
	healthBarUpdate ();
	on_q = healthUp;
	on_a = healthDown;
	on_w = healthMaxUp;
	on_s = healthMaxDown;
	while ( !key_esc )		
		wait(1);
	sys_exit ( NULL );
}


Salud!

PD: You might redo the engine tutorial about windows. I guess it is clearifier enough.