Louis,

a hunch, which might work.
It seems that, if you try to set the length in your preamble, 
`\beamer@frametextheight` is not yet defined.
So, you might try the hook `\AtBeginDocument` to see if the definition 
comes at a better timing.

     #+LATEX_HEADER: 
\newlength\mytextheight\AtBeginDocument{\makeatletter\setlength\mytextheight{\beamer@frametextheight}\makeatother}

As I said, it's a hunch, for I haven't tested. But I think it may be it.

Best,
Gustavo.


On 29/11/2018 11:07, Loris Bennett wrote:
> Eric S Fraga <esflists@gmail.com> writes:
>
>> On Thursday, 29 Nov 2018 at 09:38, Julius Dittmar wrote:
>>> Am 29.11.18 um 09:13 schrieb Julius Dittmar:
>>>> Am 29.11.18 um 08:17 schrieb Loris Bennett:
>>>>>> #+latex_header: \makeatletter\let\mytextheight\beamer@frametexheight\makeatother
>>>>>> #+attr_latex: :height 0.75\mytextheight
>>> Oh, or perhaps just a typo is involved: is it really
>>> \beamer@frametexheight or rather \beamer@frametextheight you are chasing
>>> after?
>> Also, you may need to define your mytextheight as a length and then use \setlength instead of \let.
> After correcting the typo I followed Eric's and Julius's suggestions and
> I tried adding
>
>    #+LATEX_HEADER: \newlength\mytextheight\makeatletter\setlength\mytextheight{\beamer@frametextheight}\makeatother
>
> to the beginning of the Org file, but in this case
>
>    \the\mytextheight
>
> is 0.0pt.  After that I tried adding the definition on the slide itself:
>
>    #+BEGIN_SRC latex
>    \newlength\mytextheight\makeatletter\setlength\mytextheight{\beamer@frametextheight}\makeatother
>    #+END_SRC
>
> but got the error
>
>    ! Undefined control sequence.
>    <argument> \beamer
>                       
>    l.153 \end{frame}
>                     
>    ! Missing number, treated as zero.
>    <to be read again>
>                       @
>    l.153 \end{frame}
>
> Any thoughts?
>
> Cheers,
>
> Loris
>