Yes, the recommendation for 22B is based on the off-chance of a VLAN tag being present and the observation that slightly overestimating the per-packet-overhead is "safe".
If you want to use sqm with prioritisation, you need to switch to layer_cake.qos and then make sure to properly mark your packets before cake gets hold of them. If you use snapshot builds there is the new qosify tool that allows relative easy rule based DSCP marking and prioritization (it is an alternative to sqm in that it will itself manage its own cake instances, so do not run qosfy and sqm on the same interface).
But if all you want is to prioritize the outgoing UDP packets from one specific host you should be able to configure a simple traffic rule (action DSCP classification, for Source Address of your computer) add a static DHCP lease bound to the MAC address of your computer...