Any chance the painter still has the one he used? Spray painter in the panelshop I used to work at kept a diary with every batch he mixed, date and car type/rego. He had about 10 years worth of diary's on the shelf and give him the month the car was painted he could find the formula used in a couple of minutes.
I know that different batches of the same tinters can be marginally different over the years but at least there's a good starting point.