Lakenz when you did the ping at 1423 did you get a screen like below and did you add the 28 byte header on to give you an actual mtu 1451 and that is what you should have set the mtu to although the mtu is normally an even number which is divisible by 8.
This my mtu setting i pinged 1464 and then added the 28 byte header on to give me an mtu of 1492 do you understand now.
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
C:\Windows\system32>ping facebook.com -f -l 1464
Pinging facebook.com [66.220.158.11] with 1464 bytes of data:
Reply from 66.220.158.11: bytes=1464 time=109ms TTL=242
Reply from 66.220.158.11: bytes=1464 time=107ms TTL=242
Reply from 66.220.158.11: bytes=1464 time=119ms TTL=242
Reply from 66.220.158.11: bytes=1464 time=114ms TTL=242
Ping statistics for 66.220.158.11:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 107ms, Maximum = 119ms, Average = 112ms
C:\Windows\system32>
The bytes will be 1423 in yours bearing in mind that it is usually a an even number divisble by 8.