Just a caution... I'm not sure what your application is that requires better than 5% accuracy given most home humidity controls or weather stations are more like 10%, but be sure you take the whole circuit and it's environment into account in your calculations - the nominal error of a 555 with standard components, the temperature impact on all the devices, and so on. You may quickly find that your 1% device, in the real world, actually reports worse than the 5% device. YMMV.