This is true, but also a ne needs to ensure potassium and phosphorus are also in the correct percentages.It is very, very, hard to overdose on oral calcium.
In your example of the cow with milk fever it is a delicate balance to give the calcium very slowly to cure the cow and not cause cardiac problems. But that is intravenous calcium.
Oral calcium is actually very poorly absorbed and the bulk of it just passes straight through.
It is theoretically possible to cause heart problems with oral calcium but really very, very, very unlikely.
Now I have another elderly hen laying soft eggs - I have suspected Sharpie as laying soft eggs for quite a while now, but never caught her in the act.
Today I happened to notice Mr P trying to breed her and she wasn’t making a fuss but he was not getting it done.
So after he moved on I went and picked her up and happened to notice a ‘mucky’ bum - I took a closer look and noticed membrane hanging from her cloaca - it broke off but she still had more to expel, I popped a whole calcium in her and left her to see if she would pass it on her own.
About an hr later she came sashaying out and I couldn’t see anything else hanging out from her.
I noticed that the spot she roosted on had a ‘broke’ egg liquid, so I guess I’ll put her on calcium also now.
In the upside Larry laid a beautiful brown egg today

Haven’t seen Petunia laying anything in a couple days, but she only lays every few days.
I caught Tippy laying a lovely pink egg and Eli-too a beautiful blue one.