Waterproofing additive for concrete/render

I am in the process of building a small (600 litre) raised fish pond in my garden, I have built the block work walls and now its time to lay the floor slab, I have hand mixed loads of concrete previously for my garden and building projects but never added waterproofing admix.

The instructions say to add 250-500 ml to 50kg of cement, so using my usual mix of 3 parts gravel, 2 sand & 1 cement measured using buckets it works out i should add between 35-70 ml to the mixing water then to the concrete mix (the cement weighs 7kg per bucket)

I am going to add 70 ml but this just doesn't seem enough to me.

Thanks for any responses.