Only laying chickens need calcium, well because an eggshell needs alot of calcium. Roosters and chicks don't need that much calcium. Most feeds already contain about 1% calcium.
20% protein is fine.
There are two types of limestone that come from the ground:
Calcitic limestone. This is about 95% calcium carbonate which is 38% calcium. This is the same as oyster shells and egg shells. Fine for layers. Not real common so if it doesn't say "high calcium" or "calcitic" then it probably isn't.
Dolomitic limestone. This is about 50% calcium carbonate (20% calcium) and 40% magnesium carbonate (11.5% magnesium). Magnesium cometes for absorption so less calcium is absorbed. Probably fine to feed temporarily but not long term.
Of course no quarry is the same so there are variations in calcium carbonate and magnesium carbonate.