It looks like you might be going with Tyler's method but, to answer your question:
interesting, i never knew about this function before. How should i code it?
You code it exactly as I typed it:
min(spawnTime + rateChange, maximumSpawnTime)
Replace rateChange with the amount you want to change the variable with. In your picture you are just adding 1 so, replace rateChange in the above with 1. Replace maximumSpawnTime with the maximum you want the spawnTime to be. If you don't want the spawnTime to be higher than 5, replace maximumSpawnTime with 5. Other than that, you can copy and past this equation directly into where you are setting the variable. Again, use the "Set Variable" method, not the "Add To" variable method.
I hope that clears things up for you and good luck with your project.