Because the detect level for a digital input is so very unknown - and so very much affected by temperature - one way to get any control of the situation is to build a design where you also have a capacitor of known value. Performing the same operation with this capacitor will allow your processor to recalibrate with a moving input threshold value.

The above isn't so fun, which is why you should use an external comparator instead. Then you can use two 1% resistors to form the detect level that you should wait for. Not perfect, but much better.