Cascade -90 ° C

Greetings to all
I retrofitted a unit cascaded to the second stage with a final temperature of -90 ° C. the system had a load of 1 kg with 0.1 kg R503 R12.
I replaced the oil with oil poe, and primed, 1 kg 0.1kg R23 with R134a.
after the test run the final temperature reaches -65 ° C.
can someone help me with similar experiences?
which you think may be the right mixture of refrigerant retrofit?
Thank you.

experts please help me.

& thanks in advance

With R134a is quite impossible to reach -90°C.
You had to use R23, but then again it is impossible to say, unless you give complete description of components, exchangers and compressors of the two stages.

You can use ethylene for 2 steps, if you find)

Thanks for the answers
What do you think if I use the refrigerant R508B?