There is no advice. Forms don´t accept formulas yet.
Thus my method won´t work… and the Grist method created 2 months later won´t work either.
Yeah, I hope Grist team implements formulas on the Forms soon.
My only advice is to not use forms when you need formulas… must try to use Cards.
I see some people probably new to Grist look the forums on ways to create dynamic reference drop downs in Grist, find this thread and like it.
BUt it’s important to remember some two months after I created this thread, Grist Team created a native method to Grist to do the same thing, in much simpler and more effective way.
Check the this Grist Help section
I got here from google too, this thread is above the official documentation in the results 
could a moderator possibly edit the original post with a pointer to the docs for the native functionality? so future readers don’t waste time reading through the thread. (here is the link again for the ultra lazy)
