Their «time machine» is essentially just a numerical approximation of an immense volume of air trapped in a large, transparent box with the open ocean at the box's base; the computer simply calculates how the constituent gases in the box should react and mix with each other over time. (scientificamerican.com)
However, the above two equations are sufficient to do a numerical approximation in the spreadsheet of your choice. (mechanics.stackexchange.com)
value shown in my comparison table for each fund is a numerical approximation of the average credit quality rating provided by Morningstar (M *), but with some adjustments applied. (kevinoninvesting.com)