The "8 point grid" concept is about choosing a base unit size. But that only gets you so far (for the record, I prefer 6). The real underlying issue at question in such systems is the ...
The most widely known and well-developed example is Le Corbusier's Modulor which he called the harmonious scale.
Le Corbusier's scale was influenced by da Vinci's Vitruvian Man exploration (below) and they both carefully studied Vitruvius' pivotal work De Architectura.
As you can see from the diagrams, The Modulor and Vitruvian Man sought to find the ideal proportional scale by analysis of idealized human proportions.
Another key concept of The Modulor and Vitruvian Man is the Fibonacci Sequence and the value it progressively approaches: the Golden Ratio (1:1.618...). The Golden Ratio has been observed in various naturally occurring systems (such as the spiral arrangements of sunflower seeds and pine cone scales, and various idealized aspects of human form). It has been cited as a pervasive example "harmonious" proportion in various art and architecture writings for centuries.
The goal in these systems is to identify a harmonious series of numbers that follow a given ratio. This series then becomes the foundation of all dimensional decisions.
A simple linear scale (eg,
n + 8) produces a sequence of units whose differentiation from one to the next is unrelated to the progression as a whole. Dis-proportionate progression, if you will.
Using a logarithmic scale (eg,
n * 1.618) the relationship between points on the scale becomes proportional to the relationship between all other points. Because this progression becomes quite rapid from a linear perspective, Le Corbusier promoted the dual scale seen in the sort of spiral-like measure in his diagram.
Defining one or two proportional scales for a project will provide you with a referential system of order. This is particularly evident as you specify type sizes, but you can also apply this to image sizes, buttons, the layout grid, or anything else you can dream up.
A single, experienced designer may find that they gravitate toward this sort of proportionality naturally. Such a designer might even be able to do so very rapidly. But as soon as you introduce a second designer to that equation or expect a developer to implement designs, having a system to constrain decision points can be very helpful.
For example ...
Here's a "dual strand" scale applied to type sizes and buttons. The scale is roughly 1:1.33 with some rounding adjustments for ease of use. The bold red numbers were the starting point (based on desired body font size and minimum button height). Numbers in grey are part of the scale, but not used here.
Arrange this system within your [desired base unit] grid and you have a pretty handy reference system.