I am building my 18x36 in groud pool and had a question regarding the 2" screed coat on the bottom. I think that I want to mix my own rather than purchase pre-mixed bags of screed mix. I have seen that there are two real options with the first being cement + vermiculite, and the second cement+vermiculite+mason sand. I think I like the latter mix better as it will tend to thin out the vermiculite and cure a bit harder. Does anyone know the mix proportions for the cement+vermiculite+mason sand screed bottom? I believe I should be looking for the medium vermiculite, correct?