Calculate new SUBUID range:
PREVIOUS="$(sort -t: -k2 -n /etc/subuid | tail -n1)" && PREVIOUS="${PREVIOUS#*:}" && echo "$PREVIOUS"
165536:65536
FIRST="$((${PREVIOUS%:*}+${PREVIOUS#*:}))" && echo "$FIRST"
231072
NEW=${FIRST}-$((${FIRST}+65535)) && echo "$NEW"
231072-296607
Then use that range for the new podman user:
sudo usermod --add-subuids 231072-296607 --add-subgids 231072-296607 USERNAME
Propagate changes: podman system migrate
And validate as the new user: podman unshare cat /proc/self/uid_map