If for example I am building a stud wall or cutting into an existing wall for a new doorway I would start with the door size, say for example 2'6".
A door lining leg is 1 1/4", two legs are 2 1/2" thick in total. I would then add a further 1/2" or 3/4" for fittin tolerance and a gap between the door and lining.
The height of a door is 78". From finished floor to inside the top of the lining is 79" to allow for carpet underneath the door and a small gap above. Lining is 1 1/4" thick so in total from finished floor to bottom of studwork at least 81".
For an external door I would add 10-15mm extra to the frames overall width and height for the opening size.
If that's not clear or I misunderstood the question let me know why i'll try again!
