Hello, you can make it by this algorythm:
1) Write chat command:
/SpawnPointShowPos {number of preset} {Monument name}
/SpawnPointShowPos 4 Launch Site
the name is provided in the BetterNpc data file in the format: oxide/data/BetterNpc/Monument/Launch Site.json
number of preset u can find by counting, example, "Names" parameter in this file
2) After you find a specific NPC in the game, its number will indicate the ordinal number of coordinates in this preset.
"Own list of locations (not used for Road and Biome)": [
"(203.7, 3.0, -22.6)", ( 1 )
"(192.6, 3.0, -22.5)", ( 2 )
"(198.4, 2.9, -7.2)", ( 3 )
...etc.
3) after finding the right line, you can either delete it and abandon this spawn location, or experiment with the second coordinate to lower it to the position you need.
In the current version of the plugin, each time data files changes, BetterNpc must be reloaded to apply the changes. I hope that this information is enough for you, if not, please specify what you failed at.