Everything sounds pretty good. I assume you used a total of 1000ml water. Everyone works a little differently...I use Knox as my standard gelatin, but I use a bit over 110 grams per liter (11%), but the rest is about what I use..

Anyway, your tissue is not dry yet. Once set, I remove the tissue from where I pour it and tack it to a piece of cardboard. Then stand it upright (less chance of dust falling on it) and have a fan blowing air across it over night. If I do not use a fan, then it will still be rubbery like yours. After the fan over-night, I let the tissue air dry for a total of 48 hours before I use it. I am usually at 60 to 75% humidity, but rarely over 20C.

The surface has to be dry to the touch -- but as important, dry below the surface...which is why I wait the full 48 hours before use.

What are you using for your tissue support material? I use a material the does not pass moisture...the tissue can only dry from the one surface. If you use a pourous material, then the drying times can be a little shorter.