Split arc archives by size ??

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try to increase threshold percent. if you increase it to 100,it creates one part.in this picture threshold is 30 so it have 15gb in one part which means %30 of the all data. 50.30/100=15gb and volume size is 4473 so it compress 15gb data and split it every 4473mb.
maybe precomp is gets error.it can't work all files it's beta program try to remove it and 0.42 gets more error 0.38 more stable a think if you are use it, use 0.38v.