I'm familiar with the 97 UBC equation for calculating horizontal 'g' value but if the vertical 'g' is also required, what factor should be applied to the calculated horizontal value?

Here's a quote from "Guidelines for Seismic Evaluation and Design of Petrochemical Facilities" page 4-17:
"If vertical earthquake forces are to be considered explicitly in design or analysis, they can be obtained by scaling corresponding ground horizontal accelerations by a factor of 2/3" but higher values may be required if you are close to the seismic fault.
ASCE7 & IBC set the static verical component to 20% of Sds - the short period design response acceleration.