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