Took Copilot's advice and defined some magic numbers

source/affine_background.c

@@ -7,6 +7,10 @@ BG_AFFINE bgaff;
 AFF_SRC_EX asx = {32<<8, 64<<8, 120, 80, 0x0100, 0x0100, 0};
 static uint timer = 0;
 
+#define TOP_SCANLINE_OFFSET 228
+#define ANIMATION_SPEED_DIVISOR 16
+
+
 void affine_background_init()
 {
     memcpy32_tile8_with_palette_offset((u32*)&tile8_mem[AFFINE_BG_CBB], (const u32*)affine_background_gfxTiles, affine_background_gfxTilesLen/4, AFFINE_BG_PB);
@@ -23,21 +27,21 @@ void affine_background_hblank()
     // This fixes a visual issue where the top 3 pixels of the scanline are out of sync
     if (vcount >= 160)
     {
-        vcount -= 228;
+        vcount -= TOP_SCANLINE_OFFSET;
     }
 
     const s32 timer_s32 = timer << 8;
     const s32 vcount_s32 = vcount << 8;
     const s16 vcount_s16 = vcount;
-    const s32 vcount_sine = lu_sin(vcount_s32 + timer_s32 / 16); // dividing the timer by 16 to make the animation slower
+    const s32 vcount_sine = lu_sin(vcount_s32 + timer_s32 / ANIMATION_SPEED_DIVISOR); // dividing the timer by 16 to make the animation slower
 
-    asx.scr_x = 128; // 128 on x and y is an offset used to center the rotation
+    asx.scr_x = (SCREEN_WIDTH / 2); // 128 on x and y is an offset used to center the rotation
-    asx.scr_y = vcount_s16 - 128; // scr_y must equal vcount otherwise the background will have no vertical difference
+    asx.scr_y = vcount_s16 - (SCREEN_HEIGHT / 2); // scr_y must equal vcount otherwise the background will have no vertical difference
     asx.tex_x = vcount_sine;
     asx.tex_y = vcount_sine / 64;
     asx.sx = vcount_sine / 32;
     asx.sy = vcount_sine / 16;
-    asx.alpha = (vcount_s32 + timer_s32) / 16;
+    asx.alpha = vcount_sine + (timer_s32 / ANIMATION_SPEED_DIVISOR);
     
     bg_rotscale_ex(&bgaff, &asx);
     REG_BG_AFFINE[2] = bgaff;