In mathematics, modular arithmetic (sometimes called clock arithmetic) is a system of arithmetic for integers, where numbers "wrap around" upon reaching a certain value—the modulus.

The Swiss mathematician Leonhard Euler pioneered the modern approach to congruence in about 1750, when he explicitly introduced the idea of congruence modulo a number N.[1]

Modular arithmetic was further advanced by Carl Friedrich Gauss in his book Disquisitiones Arithmeticae, published in 1801.

A familiar use of modular arithmetic is in the 12-hour clock, in which the day is divided into two 12-hour periods. If the time is 7:00 now, then 8 hours later it will be 3:00. Usual addition would suggest that the later time should be 7 + 8 = 15, but this is not the answer because clock time "wraps around" every 12 hours; in 12-hour time, there is no "15 o'clock". Likewise, if the clock starts at 12:00 (noon) and 21 hours elapse, then the time will be 9:00 the next day, rather than 33:00. Since the hour number starts over after it reaches 12, this is arithmetic modulo 12. 12 is congruent not only to 12 itself, but also to 0, so the time called "12:00" could also be called "0:00", since 0 ≡ 12 mod 12.
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Here are some references for modulo in its non-arithmetic meaning.

modulo
2. (Par extension) (Familier) Non prise en compte (utilisé exclusivement comme apposition pour signifier « sans prendre en compte », « en négligeant », « à [ce qui suit] près » ; voir les exemples ci-après).
• On arrive aux mêmes conclusions pour les pays limitrophes modulo la nature des biens exportés.
• En général, quand j’arrive chez moi après une longue marche, je ne suis pas mouillé modulo le temps qu’il a fait.
http://fr.wiktionary.org/wiki/modulo

[...] the word modulo has acquired several related definitions with time, many of which have become integrated into popular mathematical jargon.
Generally, to say:
• A is the same as B modulo C
means, "more-or-less", as in:
• A and B are the same except for differences accounted for or explained by C.
http://en.wikipedia.org/wiki/Modulo_(jargon)#Example

(Note that in these examples – as in the source text – a number does not follow "modulo.")

The link between the arithmetic and figurative definitions of "modulo" is explained in the following Slang Dictionary entry:

modulo
prep. Except for. An overgeneralization of mathematical terminology; one can consider saying that 4 equals 22 except for the 9s (4 = 22 mod 9). "Well, LISP seems to work okay now, modulo that GC bug." "I feel fine today modulo a slight headache."
http://dictionary.reference.com/browse/modulo

So we have "non prise en compte" (approximately, "without taking into consideration"] from the French Wiktionary entry, and "more or less" or "except for" from the English Wikipedia and Slang Dictionary entries.

But it turns out that we're revisiting territory that has already been explored previously on the KudoZ website in a thread for "modulo pour les tickets repas." tragedyqueen selected a response from "except for luncheon vouchers" and entered "without taking luncheon vouchers into account" into the glossary while noting, in a Grading Comment, "I found a definition for "modulo" = sans tenir compte de." http://www.proz.com/kudoz/french_to_english/human_resources/...

Of course, it's also possible to use "modulo" itself in English, just as in the French. In such a case, since no arithmetic base is involved, the text would be informed by the word's figurative meaning. However, such meaning is often unclear even to native English speakers. Witness, for example, the extended discussion of the OED entry on the languagehat website: http://www.languagehat.com/archives/002875.php

Best of luck with the text, Louisa...
And Happy New Year to all. :D