How should we translate double negatives/negatives that are unnatural in English?

Question

Is there a basic rule for translating negatives/double negatives that do not feel natural in English. The example I have been struggling is as follows:

[TOPIXは29年ぶりの安値に沈んでいます。]この株価水準は、経営者が暴走し、保有する資産を全く活かせずに、毎年赤字を垂れ流すという前提でなければ、説明のつかない水準です。

[TOPIX has fallen to a 29 year low. ] On the basis that management are not in disarray, assets are being properly used and losses are not being annually hemorrhaged this level cannot be explained.

I can think of several ways to express the same view but ultimately (if it is not dumb question) should we just choose what we think is the closest natural equivalent?

Bonus: I have taken a liberty with 暴走, which is not really "disarray" but seems to fit better. Is this commonly accpetable practice?

Answer 1

The way I would do it here is to use "unless":

[TOPIXは29年ぶりの安値に沈んでいます。]この株価水準は、経営者が暴走し、保有する資産を全く活かせずに、毎年赤字を垂れ流すという前提でなければ、説明のつかない水準です。

[TOPIX has sunk to a 29-year low] The stock price level is unexplainable unless there is the assumption of managers being out of control, absolutely failing to make the best use of the assets which they possess, every year running deficits.