Hello all - I wonder if I could pick your collective brains, please?

I'm about to take on a commission for a simple pendant to be made, for sentimental reasons, from 2 x 9ct rings (hallmarked and acid tested) that my customer has inherited. I don't work with gold but would like to start and am keen to push myself out of my comfort zone now and then!

The customer is fully aware that re-using metal has its pitfalls and isn't always recommended but would still like to go ahead. I'm now at the stage of pricing the job and seem to remember reading that it's best to add some fresh gold to the mix when melting down old metal in order to maintain the carat/purity. If this is the case (and not something that I've imagined!) what ratio of old to new is recommended and should it be the same carat, or higher?

I've spent hours online reading up on this today and can't seem to find a definitive answer!