For my first kitless I started with the front section and the hole diameter for the ink converter to fit in the back and the nib unit from the front. Those body threads had to have a minor dimension the converter would fit through. Also check the diameter of the hole in the back of the front section and make sure when you cut the same thread you have enough wall for strength.
For the cap you make sure your front unit and nib is smaller than the tap drill for the cap thread. Now check that when you turn the same cap thread on the body you will have enough wall thickness between the body male thread and the female front unit thread. If not go up a size.
Look online for a thread calculator that you can put diameters and pitches to look at the dimensions to start stacking them up on paper. The size of the front section and nib drives the rest of the pen.
For a Bock #5 my front section to body thread is 9mm x .75. It leaves enough wall at the rear for the converter using metal or acrylic. With a metal bodied pen I can use a 11mm x .75 thread. This would result in a wall thickness too thin for acrylic and 12 mm would likely be required. At 11mm x .75 my front unit has to be smaller than 0.400 inches. In metal I can use these to make a pen at 0.50 inches in diameter. An acrylic pen would have to be larger and a #6 nib would drive everything larger.
Also check out this:
http://content.penturners.org/library/general_reference/taps_dies_kitless.pdf