same distance of watermark text independent of DPI

watermark.py

@@ -81,8 +81,10 @@ def create_watermark_template(img_file, matnum, dpi):
     # Positions of watermarks
     x_pos = []
     y_pos = []
-    for y in range(0, height*2, 500):
-        for x in range(0, width*2, 500):
+    x_dist = int(2*dpi)
+    y_dist = int(2*dpi)
+    for y in range(0, height*2, x_dist):
+        for x in range(0, width*2, y_dist):
             x_pos.append(x)
             y_pos.append(y)