You`re dead right Stringfellow, its not an exact science, in fact calling it science is doing an immense disservice to proper scientists 
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I just happened to have some fairly dry, used potting compost and used it to bulk up the sand. It did make the mixture a bit dryer and as mentioned previously you cannot afford to have it too damp. Having said all that, I doubt that there was any more than 20% compost.
