'Rubbery' certainly sounds like too much rennet. It is really hard to measure a small amount of rennet from a tablet, particularly if you are using something like Fromase, where a single tablet is good for 50 litres. Best way is to break it into half, or quarter, and mix that with a measured amount of water. You then use only as much of the solution as appropriate for your recipe, and throw away the remainder. I believe dissolved rennet only lasts a short while (an hour maybe) before it starts to degrade. Good luck!