Help with wholemeal + white flour recipe

I'm new to this and I don't really know what I'm doing so please bear with me. 

I recently did my basic white bread recipe for my Kenwood bm250 machine (which has not been very consistent), and I ran out of strong white flour so I substituted a bit with wholemeal. It came out tasting very good but also "floppy".

The bread was nice and soft (it's usually quite dense), not doughy or undercooked in the middle, just the way I like it except for the fact that it's really, really soft and each slice "floppy" and limp. The crust is really good.

Here is the recipe for a 750g loaf:

290ml          water

1 1/2 tbsp    Vegetable oil                                                   I use 22g butter

450g           Unbleached White Bread Flour                        -I use strong white bread flour

4tsp            Skimmed Milk Powder                                       I omit this in all my recipes.

1 1/2tsp       Salt                            

1 tbsp          Sugar

1 1/2tsp        Easy blend dried yeast. 

According to the manual, 1tsp is 5g and 1tbsp is 15g. 

I substitute vegetable oil with butter and I don't use milk powder much. According to my prior research, it's fine and previous recipes I've made have not had any issues with my using butter instead of oil or omitting milk powder for basic breads. 

For my wholemeal + white flour, the first time I think I used around 60g wholemeal and 390g white flour meeting the total 450g white flour that was stated in the recipe.  I thought the floppiness was due to me using a bit more water (310 instead of 290g) but the second time I used the exact amounts of everything and it still came out "floppy". 

The second time I think I used 80g wholemeal to 370g white flour. 

I really like the taste of this bread, I'm hoping someone can help me with the recipe to figure out what's making it this way.