Hi Anthony,
Sarah is correct, spread distance is accumulated over time for each possible source and destination cell combination. ST-Sim uses the age of the source cell as a proxy for how much time has elapsed and hence how many draws to make from the spread distribution for each source/destination vector. For this reason, the initial age of the source cells is important. In your simulations you will see spread happening soonest to neighbors of the oldest source cells. Spread distance is calculated from center to center. When this distance threshold is exceeded, the destination cell will transition.
Regards,
Leonardo