Replace magic values with proper variables.

AM2RLauncher/AM2RLauncher/MainForm/MainForm.Events.cs

@@ -682,9 +682,9 @@ namespace AM2RLauncher
             // Get drawing variables
             float height = drawable.Height;
             float width = drawable.Width;
-            //TODO: apparently winforms is the big outlier here. Works normal on wpf, I have *no* idea why, investigate and submit issue at eto
-            float scaleDivisor = OS.IsWindows ? 955f : 715f; // Magic brute-forced values. Don't ask questions, because we don't have answers >_>
-                                                                    // Also, seems like nix systems share the same scaleDivisor. Again, don't ask.
+            //TODO: apparently winforms is the big outlier here. Works normal on wpf, I have *no* idea why, seems related to our image. issue has been submitted at eto
+            float scaleDivisor = OS.IsWindows ? formBG.Width : formBG.Height;
+
             float scale = height / scaleDivisor;
 
             // Do the actual scaling