Learn English – How to write a range involving negative numbers

According to The Chicago Manual of Style^†, section 14.21, this is the normal way to do it:

A note number should generally be placed at the end of a sentence or at the end of a clause. The number normally follows a quotation (whether it is run into the text or set as an extract). Relative to other punctuation, the number follows any punctuation mark except for the dash, which it precedes. (emphasis added)

In my experience (which is mostly from reading linguistics papers), this is the usual practice. I decided to check my memory, so I opened ten papers at random on my hard drive by different authors. I found that nine followed this practice, while the remaining paper placed the superscript reference number before any punctuation. So it seems (at least from a sample of linguistics papers) that this practice isn't followed universally, but I'd nonetheless recommend you follow common practice both for aesthetic reasons and to avoid distracting the reader.

^† Thanks to Jason Patterson for pointing this out in the comments section.

1. When discussing with customers and / or suppliers, it is highly recommended to use formal communication.

2. Regardless of the type of communication (formal or informal), sentences which start with numbers look awkward. There is always a way to form a sentence with numbers, and avoid having a number as the first element.

Exceptions:

• the numbers are written as text (two instead of 2);

  Three Men in a Boat

• you actually present a formula, and there is no real sentence

You can use any article / book as a reference. I do not remember any of them starting sentences with numbers represented as digits.