Yeah, thanks for bringing this to my notice,

For a 100m2 piece of land you need 10kg of calcium, that should be 100%.

But from our question, Marl has only 50% calcium, so to make that up to the required dosage, simply multiply the average recommended dosage of calcium for a 100m2 land by 2
(I.e 10kg X 2) this will give 20kg of Marl. This is so as to make up to the requirements of 100%calcium.

Therefore, the required dosage of Marl is 20kg for a 100m2 piece of Land.

Thanks.