Use the same default ground precision whatever the base zoom is.

geojson.c

@@ -20,7 +20,7 @@
 #include "projection.h"
 
 int low_detail = 10;
-int full_detail = 12;
+int full_detail = -1;
 
 #define GEOM_POINT 0                /* array of positions */
 #define GEOM_MULTIPOINT 1           /* array of arrays of positions */
@@ -581,6 +581,13 @@ int main(int argc, char **argv) {
 		}
 	}
 
+	if (full_detail <= 0) {
+		// ~0.5m accuracy at whatever zoom
+		// 12 bits (4096 units) at z14
+
+		full_detail = 26 - maxzoom;
+	}
+
 	if (outdir == NULL) {
 		fprintf(stderr, "%s: must specify -o out.mbtiles\n", argv[0]);
 		exit(EXIT_FAILURE);