Getting a nice += in LaTeX math

I am working on an appendix for a paper that uses MCMC, and I decided to document some change of varible calculations in the interest of reproducibility (they are quite complex, because of multivariate determinants). But how can I typeset them nicely in $\LaTeX$?

\mathtt{target} += J_f

gives $$ \mathtt{target} += J_f $$ which is to be expected, as + is a binary operator and = is a relation, so $\LaTeX$ is not expecting them to show up this way.

We can remedy this as

\mathtt{target} \mathrel{+}= J_f

which shows up as $$ \mathtt{target} \mathrel{+}= J_f $$ which is an improvement, but is still not visually appealing.

Making the + a bit smaller with

\mathrel{\raisebox{0.19ex}{$\scriptstyle+$}}=}

yields $$ \mathtt{target} \mathrel{\raise{0.19ex}{\scriptstyle+}} = J_f $$ which looks OK enough to preclude further tweaking. Note that MathJax does not support \raisebox, but you can use

\mathrel{\raise{0.19ex}{\scriptstyle+}} = J_f

which renders the as above.