I'm going to present a slight frame challenge. Symbols such as -, +, ×, ÷ are best used in formulas, since the context makes their meaning clearer.
If you're using prose, I'd suggest phrases such as "the difference between total widget count this month and last month," or "the sum of total widget and contraption counts," or "the product of hours worked and the number of people working," etc.
This is largely my opinion. I did find a mathematical style guide from Virginia Commonwealth University with this advice (p. 2):
Avoid misuse of symbols. Symbols such as =, ≤, ⊆, ∈, etc. are not
words. While it is appropriate to use them in mathematical
expressions, they are out of place in other contexts.
The Chicago Manual of Style (16th edition) gives different guidance (p. 581):
Words versus symbols in text. In general mathematical symbols may be
used in text in lieu of words, and such statements as "x ≥ 0"
should not be rewritten as "x is greater than or equal to zero."
Nonetheless, symbols should not be used as a shorthand for words if
the result is awkward or ungrammatical.
Personally, I feel like Chicago may be presenting a false dichotomy. Merely substituting the phrase "greater than or equal to" for the symbol ≥ can sound amateurish. Same for a mechanical replacement of the minus sign with the word "minus." There are more prosodic expressions such as "x is at least zero" that could be used instead.
Which takes me back to the frame challenge: If the meaning of the minus sign could be confused with a hyphen, then consider use of plain prose. Don't feel like you must translate your phrase word-for-word, merely substituting the word "minus" for the minus sign. For example, instead of trying to substitute the minus sign directly, you might express your first example as "The number of widgets remaining after accounting for theft..." or something similar.
Finally, if you are intent on using formulas because you feel they capture a complex relationship succinctly, then don't go half-way. Use actual formulas. Something like:
A = B - C
- A is the number of widgets we have on hand
- B is the number of widgets we purchased from Acme on April 7.
- C is the number of widgets lost to theft or that failed quality-assurance tests.
Or at least make it clear that you are specifying an equation by using an equals sign and putting the equation on a line by itself:
(Amount of Widgets) = (Total Widgets purchased) - (Widgets Lost)