openvpn over UDP over IPOE (DHCP/EAP) ; 1500 - 69 = 1431 bytes MAX-MTU
openvpn over TCP over IPOE (DHCP/EAP) ; 1500 - 81 = 1419 bytes MAX-MTU

openvpn over UDP over PPPOE ; 1492 - 69 = 1423 bytes MAX-MTU
openvpn over TCP over PPPOE ; 1492 - 81 = 1411 bytes MAX-MTU

https://forum.mikrotik.com/viewtopic.php?t=194872