Hi Sander
Sorry you haven't got any help with your question. I do think Tetenal products are generally available around the world.
Sadly I have not processed C41 for a number of years but I used to split both Tetenal and Kodak C41 kits in bigger sizes than 1L as well as RA4 kits from Kodak and Agfa. The issue you raise troubled me often; so much so that I eventually started to measure the contents of each new bottle before use and while the volumes were mostly close to correct, I found many instances of significant unders and overs.
The advantage of measuring each bottle and dividing by 2 or whatever number of batches you are making is that you then compensate for any error in your own measuring device. You just have to be careful about introducing too much air into the developers with the additional measurement.
Funny thing is I could never once detect any difference in the developed film or paper when using a kit that had an incorrect amount of fluid in one or more bottles. Who knows?