I hope to clarify the calculator as there are many questions on that subject. I see you have the metric to US conversion down.
I don't know of a metric version...I do find metric a lot easier...we have been going to the metric system ever since I was in grade
school...long time ago.
The calc is just a general guideline for various openings, volumes. It uses the firebox as the "reference" , but the consensus
here is to reference all calculations to the cook chamber size [since it it usually fixed tank or vessel]. That being said, put your numbers into the calc to put the FB (firebox) @ 100% or close to. I see you are at 103% that is good enough. All the openings, volumes with FB @ 100% would be your build to dimensions (or a bit larger...this is where the interpretations / opinions enter the discussion).
FB size, most of what I have read is that the FB can be larger and no affect the operation. This size can be anywhere from 100% to 130% depending upon your preference. Definitely allow some room for ash deposit and clean out.
Smoke Stack - this is length is above the CC (cook chamber) top. I intend to make mine a bit longer and shorten if necessary.
After you have all your numbers plugged in, draw it up to determine the reverse flow (RF) placement height and size of CC opening.
Here is a thread where Dave O answered a lot of questions for me. http://www.smokingmeatforums.com/t/138483/firebox-and-rf-plate-questions
There are numerous mathematical calculators on the web to figure out the area or volume of odd shaped objects. Just apply a bit of logic or post a question here.
Most important think straight line air and heat flow, reduce friction. Place your air intakes in-line with the flow. After all air is a fluid and heated air with smoke cooks some great food.
Merry Christmas and Best Holiday Wishes from the USA