if this 100GB is your daily ingesting amount of data, then this is the license size which you need to buy. Or actually you should by something like 100GB * x% to get some additional space for total license size.
There is no need for you to calculate those other. And actually you couldn't even choose those as Splunk is defining those based on your daily ingesting data.
You should just connect to your local Splunk Partner or directly to Splunk and asking quota for this.