In order to pack islands to a non-square texture follow the procedure:

  • unwrap your islands in the usual manner so they are not deformed when displayed at the unit UV square,
  • open or create a non-square texture in the UV editor. You will notice that islands will be deformed now,
  • next step is adjusting islands to the texture i.e. applying non-uniform scaling so they have correct proportions with the given texture. You can do it manually, you can also use the addon to perform scaling automatically. In order to do that select all islands you want to adjust and click Adjust Islands To Texture button. You will see that islands will have correct proportions after action is done (they will be no longer deformed with the given non-square texture). Note that this operation must be performed only once for the given islands,
  • enable the Use Texture Ratio option in the Non-Square Packing panel,
  • from now you can run the packing operation normally. You can pack islands multiple times and their proportions will always be correct with the given non-square texture
Non-Square Packing panel

Note that now you can use the Pack To Others option also when packing to a non-square texture. But remember that all new islands you want to add to the packing must be adjusted to the given non-square texture first (for example by using the Adjust Islands To Texture operation.

You can also undo the adjustment operation. In order to do that select given islands and click Undo Islands Adjustment button. After the action is done islands will be deformed again when displayed at the non-square texture. When you close the texture you will see that the islands will have proper proportions at the unit UV square. Note that for the undo operation to work properly the texture used at the time of undoing must have the same ratio as the texture islands were originally adjusted to.