You calculate it as the voice from above says 'minimums'.. If you can see the lights, you have the required slant range
The reality is the vertical component is so small vs the horizontal that there is no difference.
Eg Cat I ILS no lights. 60m (200') vertical, 1500m horizonal.
Pythagoras says the hypotenuse ie Slant vis is 1501m.
Its flying not rocket surgery. No need to over think these things