How far you set that pump in the well really depends on what it produces for water and the amount of draw down they measured during the pump test. Every well is different especially rock wells. A rock well has different draw down patterns than screened wells because of the different aquifer characteristics. I always leave at least 25' between the pump and the bottom of the well. Every well produces a certain amount of silt and sediment that will collect on the bottom. Setting the pump far enough off the bottom keeps the pump away from that material. If your measurements are correct i would raise that pump 30' feet and expect the same problem 10 years down the road. The material will eventually come up to where ever the material is coming from and stop but who knows where that is. It could be where its at now or it might be coming from where the casing sits in the rock. You have to keep in mind the more you raise that pump the more storage capacity you are losing from the well. I would monitor it and if it gets to the point where your water volume starts slowing down you will want to look into drilling the well out or find out where the material is coming from and get it resolved.I would measure the well with a 500' tape with something heavy on the end of it to confirm your measurements.