Mortar qty

Now the weather is hopefully on the up I want to lay paving slabs over my concrete yard area.
Working out how many slabs to buy is easy but I`m not sure about how much cement/sand I need. Anyone got a mix ratio and ready reckoner guide for this?
I`m laying 200 slabs 450mm x 450mm including 10% for wastage.
How much `muck` would I need for 5 dabs per slab?
Also whats the `fall` ratio away from the house for rain run off/drainage and is there a simple way of measuring it?
Grateful for advice.

it depends on the depth of bed you are laying, but 3 bulk bags will cover it. Dont lay on 5 dabs of mortar, use a solid bed. Dabs lead to the paving becoming loose and the pointing failing

Thermo, thanks for the reply.
I dont want to raise the heght of the patio/yard any higher than I need to so what do you think is the minimum thickness of bed I should use?
Any info on the best way to measure the fall?