Hello.
The last time I tested Assetto Corsa RPM% values weren't available. But if I remember correctly they used to work.
It does't depend on SimDash if RPM% doesn't work but it is the game that doesn't make this values available.
I have set a reminder for myself, for the week end, to check again Assetto Corsa and RPM%.
If I have an update I will let you know here.
Regards
Cristiano