If you multiply a time by 1.4, you are adding 40% to the development time. This is not too dissimilar from adding 50% per stop. In general, these type of calculations are excessive and designed to rescue film which has been incorrectly exposed. If you are pushing film to gain more speed, either for an aesthetic effect (increased grain and contrast) or to compensate for insufficient light levels, then 50% per stop is likely to be on the high side of what is required. But at least you won't end up with thin unprintable negs.
If you haven't already seen it, have a look at the push processing table that is included in the Massive Dev Chart (choose "Push Processing" from left hand menu) and you will find some useful guidelines.
Maths may not be your forte, but your skills in mathematics seem to be better than those of your colleagues. Perhaps they think 0.4 is 1/4th? I would suggest purchasing a calculator for each of them next Xmas.
Film Photography & Darkroom discussion
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