Is English not your native language? It is just as proper to say: "If you can calibrate some device accurately, then it will make measurements which are precise..." Some knowledge of position does not bar all knowledge of momentum. Perhaps this is where you are getting confused. As long as you are trying to measure both, you can, only with the accuracy being split between the two properties. If you choose to measure one property exactly, then you cannot simultaneously make a measurement of the other. If you do, the accuracy of your chosen property will suffer.