How does one read a formula with subscripts and superscripts? An expression like $\Gamma_{ij}^k$ seems to be pronounced "gamma sub i, j upper k". Is this a generally accepted usage? 
Question. Is there a quotable source for such usage?
Note that $k$ is not a superscript.  For those familiar with Riemannian geometry, perhaps a more convincing case is the Riemann curvature tensor $R^{i}_{jk\ell}$, where a similar question can be asked.
 A: Finding some answers along with your question now by a google.com web search (logged in) of pronouncing subscripts in algebra OR mathematics OR calculus [(not on the topic of combining with a superscript)]:
Surely the following reference has to be quotable for reading subscripts as "sub" [and superscript as "super"]:
Lawrence A. Chang, Carol M. White (ass.) & Lila Abrahamson (ass.), (c) 1983, Handbook for Spoken Mathematics (Larry’s Speakeasy), (c) University of California, <http://englishlangkan.com/produk/E%20Book%20Handbook%20for%20Spoken%20Mathematics.pdf> archived 2020-01-25, p. 9.
The following could also be quotable for the same, albeit not a hardcopy and possibly a bit amusing on account of the seemingly lower level of intended audience prerequisites:
https://www.mathsisfun.com/definitions/subscript.html
Addendum re critique
My initial answer (outside of square brackets above) was referred to as just a link in one comment and deletable low quality in another.
Reviewing the Q&A, I can see I in it interposed my own wondering about pronouncing $X_i$ or $f()_i$, that I had on my mind.
Seeking consensus on interpreting the question, I propose: it starts with an expression without a mathematical topic; a lack of topical familiarity or recognition of expressional patterns, makes it serving no context for me; it describes an observation of seeming customs around pronunciation; it goes on to ask about general acceptance of this seeming custom, and emphasizes in the following:

Question. Is there a quotable source for such usage?

General continuation
I realize (now) the question is about combining sub- and superscripts. My greatest hope for the questioner is that the "quotable source" I provide can be seen as valid towards the pronunciation of the parts.
My quotation of the referenced source follows (MathJax-formatted):
$$
\begin{array}{lcll}
\text{Expression}    &           & \text{Speak} & \text{…} \\
\hline
                     &\space\vdots \\
                     &           & \text{a superscript n} \\
{a^n}                & \text{or} & \\
                     &           & \text{a to the n} \\
                     &\space\vdots \\
                     &           & \text{a subscript n} \\
{a_n}                & \text{or} & \\
                     &           & \text{a sub n} \\
                     &\space\vdots \\
\end{array} \\
\text{ } \\
\text{(}\textit{SECTION III - BASIC SYMBOLS}\text{, page 9)}
$$
As for combining the parts - though it still depends on topic, context, and recognition, as much as any uncommented math formulae - relative a general context, I'd say it's a matter of comfort of the situation, and I choose to accompany this statement with another quotation:
$$
\begin{array}{lcll}
\text{Expression}    &           & \text{Speak} & \text{…}\\
\hline
                     &\space\vdots \\
{}_{n}{A}_{x}        &           & \text{left-subscript n capital a sub x} & \\
                     &\space\vdots \\
\end{array} \\
\text{ } \\
\text{(}\textit{SECTION VIII - STATISTICS AND MATHEMATICS OF FINANCE}\text{, page 34)}
$$
At risk of repetition, I say it's deducible - in general - from my "quotable source" that combination of expression is rather free in spoken mathematics, and in closing this rebuttal I include a new reference (in its unofficial state hardly "quotable"), and of that a quote:

Individual mathematicians often have their own way of pronouncing mathematical expres- sions and in many cases there is no generally accepted “correct” pronunciation.

        – H. Väliaho, 1999-02-17, Pronunciation of mathematical expressions, <http://web.archive.org/web/20131026031935/http://www.math.helsinki.fi/engl.pdf> retrieved 2014-01-28, p. 3. (First name probably Hannu)
PS
Now reading the intro to the main resource, and coming close to a rant anyway, I can't help including some pieces from the introduction (Chang, 1983, pp. 1-2):

A goal of the handbook is to establish a standard where no standard has existed, so far as I know. However, this standard represents only one of many possibilities. As a blind person, I have learned mathematics by means of others reading the material to me; so my preferences are a result of direct experience.

and

If you encounter an expression that is not included in the guide, read the expression literally, that is, read it from left to right, symbol by symbol.

