if there are leaks it might mean you mixed too thin. in which case you might need another coating.
the 1 yard piece i did turned out as waterproof as sil and right at the same weight. just thinned the mix the right amount, soaked the fabric in it till it was totally soaked (not long) and then wiped off all the excess and hung to dry.
just play around with small batches and pieces of fabric to get the ratio of thinner to sil correct, then do a tarp. i would use a scale to weigh your finished samples to see how much sil you added. i used a piece that was 1 sq yd so it would be easy to tell. test under the shower head to see if water beads through badly. you can compare it to a piece of regular sil if you want.
for the final project, i would make the tarp out of breathable fabric, set it up, stretch it really tight and rub the mix on with a sponge and work it in real good on both sides, and then wipe it off really good. then just take it down when it's dry.