1 lb N/month through the growing season and shorter height of cut (2"-2.5") will promote spreading. Use something like Lesco 24-0-11.

I didn't realize the height of cutting would have an effect on spreading. I've always thought that 3-4" height was an overall decent height for a healthy lawn to shade out weeds and all that. Is there better times of the year to promote the spreading or does it generally happen all year long ? And also should I maybe go back to a longer height for the summer heat then or just try to keep cutting at 2-2.5" ? When exactly do you consider the growing season ? I'm guessing both spring and fall but not summer ?

Spring and fall is a safe bet, best not to push the grass in the heat. As soon as it cools off you can start pushing N and lowering your height of cut, probably as soon as mid-August. Keep it high through summer as it helps with water retention in the soil and also to crowd out weeds.

High nitrogen fertilization slows rhizome production and promotes leaf production. That said, it should be well fed in the fall. Moisture and soil quality are important too. You can easily see KBG creep into mulched flower beds with ease but have a hard time spreading into hard, compacted soil.

The best way to make it spread is to keep it alive for 3 years. The first year it sleeps. The second year it creeps. The third year it leaps.