Cross check the info with that fromm a knowlegeable source e.g. This
I believe that ./Ok3ch will take you through the rest should you need further guidance.
Alternatively, just head straight to sites that have automated tools e.g http://dutycalc.auto-kenya.com/ (there are quite a number of such sites, a relevant 'google' search should suffice)
PS : All these will just give you intelligent estimates. Your (KRA registered) clearing agent should be in a position to give a firmer quotation. That's what you pay them to do anyway.