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As both Rolf and myself have found it's a matter of mocking it up and hanging it literally from the rafters. Build the major components, sling them all from a suspended beam and start measuring.
In reality close enough is good enough. You will add and remove components all the time and it will never be perfect. Besides which a wee bit of weight bias in one direction keeps the tracking system happier. My system also has a long dovetail which means if I make any major weight additions, temporary or permanent ( cameras etc ) I can slide the dovetail back to compensate.
Current all up weight on my 10" Serrurier OTA is about 11 kg and then add about 3 kg for current cameras, finders etc. Rolfs, if I remember correctly is about 17 kg (?).
Check Rolf Olsens website for the whole story. My 'copy' of his is on here.
I would tend to go with tubing of a suitable wall/diameter rather than open channel struts. My experiments with angle and channel said they flexed too much and therefore had to be thicker and bigger therefore heavier, than tube struts.
If you are building this yourself you most important thing will be accuracy in parts measurements. There are a lot of duplicate parts, Struts, angle brackets, etc so jigging and careful measuring during fabrication are extremely important.
Keep us posted on progress and if I can offer any advice let me know.
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