How to make sure I get all terrain data

I am using mission planner with plane and had a question about how to make sure I get all the terrain data. I dont fly autonomus missionsbut want to make sure I get the terrain data for RTL.Currently when I want to go to new areas to fly I create fake missions to cover the entire area I plan to fly. TO do this most effeciently I wanted to use the sinple grid oprion to make the waypoints.My question is what is the max spacing I can have for line spacing and spacing inline to make sure all the data for that area is downloaded?

Thanks Matt,
I am aware of needing to watch the Ter Load and ter Pend until it zeros out. It was more of an issue trying to figure out what the waypoint spacing needs to be to fully cover an area.I am using terrain spacing of 30 for my terrain data.I have looked for a way to load an entire state or larger area but so far have been unsuccessful with my google searches

I found topics on how to load the 10m spacing into Mission planner but noting on how to load larger areas into the terrain folder on the SD card