You don't really have to get down to the fine points of different thread standards (US vs. ISO or whatever). The basic drawing that you show is enough. But you have to know how the threads are specified. The real thing that determines a thread's diameter is the PITCH DIAMETER. It may not say this in the Handbook or any other standard, but it really is the most basic number for any particular thread. The flat at the crest and the fill at the root are both somewhat subject to variation. Of course, there is a tolerance range for the Pitch Diameter also, but it is still the basic number.

Checking a little math against a particular thread listed in Table 3 in the Handbook tells you that the the Pitch Diameter is calculated from the (nominal?) value of the Major Diameter by assuming that the flat at the crest is exactly 1/8 (12.5%) of the Height of a sharp Vee thread. This 1/8 of the Height is NOT included in the calculation so the Major Diameter of the thread is at that flat, not at the sharp Vee point.

For instance, using a 1-8 TPI thread, the Height of a sharp Vee thread is 1/8" x cos(30) = 0.1083". Then the Pitch Diameter can be calculated as MD - 2 (3/8 H) = 0.9186". This value is exactly what is shown in the Handbook for the maximum PD for that thread. My point is, the 1/8 flat at the crest is NOT part of the thread nor is it to be included in any calculations.

A similar calculation gives the MAXIMUM Minor Diameter as MD - 2 (5/8 H) = 0.8626". Now things get a bit more fuzzy. Why is that? Well, that is the Maximum Minor Diameter, but if a thread is being cut in a production situation, the cutting tool will wear. So it is anticipated that the tool will start cutting the thread deeper than that. In fact, the UNSTATED assumption is that the thread cutting tool will start cutting the root of the thread at the same 1/8 H point from the sharp Vee as the crest is specified at. How do I know this? I have no written references, but I have done a lot of checking and it is what makes sense. In fact, it is the ONLY thing that makes sense when both tool wear and the history of thread forms are taken into account. Anyway, the Minor Diameter is a very poorly controlled number in a thread form. It is, in fact the one that varies the most and you can not count on anything except the maximum value given in the Handbook. This is why I said that you do not have to sweat the fine points.

But that maximum value is not what you want for a strength calculation: you want the MINIMUM value, not the maximum one. And this is where any/all of the published formulae will not serve. You need to go to that 1/8 H value that I have stated is the real starting point for a threading tool. I can not guarantee this number, but, as I said, it is the only one that makes sense when you consider all the factors. So, foo a safe calculation of the tensile strength of a bolt you should use this formula:

Minimum Minor Diameter = MD - 2 (3/4 H). And for my 1-8 example that would come to 0.8356".

When I was talking about wear on the threading tools, that would apply to both a thread cutting tool and a thread rolling tool. Both will wear in use. I used the Class 1 values for the 1-8 thread in my example calculations. And I only calculated the maximum value of the Pitch Diameter. The minimum value for the Pitch Diameter does not follow from the minimum value of the Major Diameter: The Pitch Diameter appears to have a smaller allowed range. To be completely safe you may want to subtract twice the value of that PD range in your calculations of the Minor Diameter. Of course, these values do vary with the class of the thread.