What is a customer worth to you?

What a typical customer spends in one visit or order. Divide last year's sales by the number of orders, or pull a middling invoice and use that.

How many times a typical customer buys from you in a year. Count the orders for a handful of regulars and take the average.

The share of each sale left after what it costs you to deliver it: materials, labor, card fees. A 120 sale that costs you 66 leaves 45%.

Of the customers you had last year, the share who bought again this year. Count last year's customers, then count how many came back.

What you spend on marketing and selling to land one new customer. Take last year's marketing spend and divide it by the new customers it won.

What you spend per year on each existing customer to hold onto them: email tools, loyalty offers, holiday cards, check-in calls.

How many years of the relationship to count. Pick a span you can believe, and use fewer years if people in your trade move on quickly.

The discount rate turns future dollars into today's dollars, because money arriving later is worth less than money now. Your borrowing rate is a fair start.