Create a task based a date field and a drop down to generate a next contact date.

Create a task based a date field and a drop down to generate a next contact date.

I hope I can explain this right.

My ecommerce business has a lot of regular customers who all purchase at different order frequencies. I'd like to set up a workflow based on the last date a customer has placed an order and set a task to contact a customer, either a call or send an email after a fixed period of time. The next contact date should reset every time a customer places a new order. It's designed as a backstop to slow customer churn.

So, for example, a customer's last visit is 01-03-2025. I'd like to set a reminder to contact them in a few weeks time via a dropdown list eg 1, 2, 3 (weeks) etc. This dropdown would update the next contact date and create a task on that date. 

I've been struggling hugely with workflows in order to make this work. 
I have created 3 fields
Last Visit
Contact Frequency
Next Contact Date

But can't find a way to create a workflow based on these fields. It seems easy enough to setup a simple reminder task - say a day, week or month later, or so many days, but I want to determine the next Contact Date based on the dropdown. I don't really want to get into functions, but wondered if there's a Formula that could be used to create the follow up date.