Count how many pulses per second

No, the offset voltage is a proportion of the 5V as set by the ratio of the 1K to the 220?. It has nothing to do with Ohm's law. Yes, you can do Ohm's law calculations on it, but that would just yield you the current flowing through the combined 1K and 220?, not the voltage at the mid point.

Read Voltage divider - Wikipedia for more detail.