Whilst it doesn't give exact guidance for the circumstances being discussed, there is lots of useful information at
http://www.caa.co.uk/docs/50/SRG_FOP...SOC_Focus1.pdf
I would suggest that if the prolonged taxy time is a regular event, then the guidance should be to increase the fuel load to take this into account. Similarly, how many fuel plans ALWAYS use the longest STAR/SID?