Dynamic Quantity from custom field in recurring Invoices

Hey guys,

i have a bit tricky workflow to implement which in ERPNext i have nearly completed (after testing many other CRM/ERP), but one thing is missing. Maybe you could me:-) Here is the part of the workflow which is not complete:

  1. Customers shall get recurring invoices with two different fixed product items in it
  2. The quantity of the products shall be taken from two custom fields of the customer and updated before each recurring invoice
  3. One of the custom fields is a calculation of the other custom field
  4. The custom fields are ready and named “Konzessionen” and “Stimmen”, where “Stimmen” is calculated via a math.ceil function of “Konzessionen”
  5. The generated invoices should be saved as drafts and then manually submitted and automatically sent via E-Mail

I have everything working, the only thing i don´t get is how to populate the value of “Konzessionen” as the quantity of Product Item A and “Stimmen” as the quantity of the Product item B. Or put differently: how can i pull the values of that custom fields in “Customer” into the “Quantity” fields in an invoice?

Anybody with a hint for me?:- )

All the best and thanks a lot for reading