The short answer to this question is NO, patent infringement is not based on a percentage of how close to the original your device is.
Patent infringement is determined by comparing the claims of a patent to the accused devise. If the device has all of the elements of the patent claim, it is deemed to be an infringing device. If the accused device does not have all of the elements of the patent claim, it is likely not infringing.
There is one caveat to the above analysis, the doctrine of equivalents. Patent infringement can either be literal infringement or infringement under the doctrine of equivalents. Literal infringement occurs when a device literally infringes the claims of the patent, or in other words when the accused device literally has all of the elements of the asserted patent claim. Infringement under the doctrine of equivalents occurs when any differences between the claimed invention and the accused product are insubstantial. One way of showing infringement under the doctrine of equivalents is when the accused claim elements or limitations “perform substantially the same function in substantially the same way with substantially the same result as each claim limitation of the patented product.” (citation omitted).
Thus, the question of patent infringement is more detailed than “can I change 5%, 10%, 15%, 20%, 25% etc. of the patented product and not infringe?” The question is does the accused device contain all elements and limitations found in the claims of the patent.