If you and your family take D3 (1000mcg), which it's really a good idea in this time of Covid, give her one capsule each day with the calcium, and half a 1000mg calcium tablet is plenty adequate. Too much calcium is hard on the kidneys, although it can take weeks for some hens to respond to the calcium and get straightened out.
Lettuce is fine. Spinach and kale has oxalic acid which slows the absorption of calcium. But at eight years old, she's got tired organs, and egg laying is going to be a tough occupation. I have a Wyandotte who is now twelve and I'm holding my breath she doesn't get into the nest box like she does every damned spring and get herself bound up again with a broken egg inside. Unfortunately, there's no stopping these old gals unless you have a fancy vet who can install a hormone implant every six months at fancy costs.