How to add field for depth below stratum?

Hi,

I am new to gINT, my goal is to plot a graph that shows Data vs depth below stratum top (e.g. the ground comprises 0.2m Topsoil + 0.8m Clay and I want to plot the data at 1m bgl as 0.8m instead).

In excel, I was able to obtain such depth by looking up the geology code with the same borehole ID and subtract the sample depth by stratum top depth. In gINT, I am not so sure how it works. I imagine I can create a new field for stratum top depth next to each geology top depth, so that the "depth below stratum top" can be calculated in the sampling tab? gINT Rule might be involved or maybe I can achieve the same outcome in a simpler way? 

Thank you very much,

Krist