Fine-tuning dimensions/whitespace parameters

I often find I need to adjust whitespace lengths in my own code. Just as an illustration, I'm in the middle of trying to insert an appropriate amount of space before the 'v's in this, to make them look 'centred'.

(I won't give the source code unless it's specifically requested, as I don't think it's relevant to this very general question.) My normal method is to adjust-compile-adjust-compile-adjust-compile-etc., until things look right. This can take a long time!

It occurred to me that a better method might be to produce a PDF with one copy of the relevant MWE per page, each with a different length used. Again, a picture should make this clearer than the text:

Of course, if you are creating this thing manually each time you need to tune a length, it is clearly not worth it. Are there any packages that would make it easy to do this?

(For what it's worth, this question is inspired by the AvsP editor for the video scripting language AviSynth; that sped up many workflows by an order of magnitude by letting you adjust parameters using sliders, rather than editing values in the script. I thought sliders were a little too much to ask for here, so this question is based on an older AviSynth technique of "animating" parameters by giving a parameter a slightly different value in each video frame.)

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You seem to want to centre a v between two (decorated) boxes. The usual way is

\fbox{one}\hfill v\hfill\fbox{two}


Not sure what other hints one could give without some more context.

If you do want to experiment with lengths then stick the whole page layout in a macro so

\def\test#1{%
\clearpage
\noindent\fbox{one}\rlap{\hspace{#1}v}\hfill\fbox{two}%
\clearpage}


then

\dimen@=0pt
\loop
\test\dimen@
\ifdim\dimen@<100pt