RogMoe:
I have however no system to tell how much water is in the tank. After searching the Net I found a few solutions.
There is a great swag of level devices available on the market for rainwater tanks but, as far as I'm aware, none use a pressure sensor. This is probably because it is not such a good idea.
In the domestic arena at least, I submit this is a rather pointless exercise in the sophisticated aquisition of useless information. The most anybody needs to know, if anything at all, is an alarm to tell when the tank is down to its last couple hundred litres, and there are plenty of drop-in float switches that are good for that. Common practice is to use a rainsaver to provide automatic town supply backup, thereby removing the need to know the level in the tank.
If you want to get technical, it may be possible to use an Arduino to monitor the performance of the rainsaver. I don't know what you would do with the result, but it might help you identify which stable door to close after the horse has bolted.