This usage is never heard in everyday English, but is fairly common in legal English and mathematics. It results from the need to translate mathematics into English in contracts and other legal documents.
A proportion is a ratio, i.e. a a vulgar fraction, in which the numerator "bears to" the denominator." So if "Insured Fixed Charges" = 10,000,000 and "All Fixed Charges" = 15,000,000 then the proportion of "Insured Fixed Charges" to "All Fixed Charges" is 10,000,000/15,000,000 = 2/3. In other words, 10,000,000 bears to 15,000,000 as 2 bears to 3.
So here is the breakdown of your passage:
adding to the Net Profit the amount of the Insured FixedCharges Take the net profit and add to it the Insured Fixed Charges
or if there be no Net Profit the amount of the Insured Fixed Charges It is equivalent to pointing out that Net Profit + Insured Fixed Charges = Insured Fixed charges when Net profit = 0, but also confirming that when Net Profit is negative it you may not subtract it from Insured Fixed Charges.
less that proportion of any loss from business operations as the amount of the Insured FixedCharges bears to all Fixed Charges.
In case the Net Profit is negative (i.e.there was a Net Loss) then subtract from the figure determined above the Net Loss times the Insured Fixed Charges divided by All Fixed Charges. That is to say the proportion, not the whole amount.
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