The Shapeoko 3, 4, and Pro use GT2 belts for the X-axis and Y-axis. This particular line of Gates belts is designed for power transmission and precise positioning and easily exceeds in specification the typical operational loads for the machine.
To optionally dial your machine in past the default settings, it is necessary to calibrate the number of steps in Grbl to match the physical dimensions of the belts as tensioned on your machine. Doing so will require precision measurement tools such as a dial indicator or pair of good quality calipers.
To simplify the math and reduce rounding errors, you may wish to set your initial values for # of steps to round numbers. The Shapeoko 3’s defaults are well-suited for this:
Mount a piece of stock which is dimensionally stable and mill 3 features in it so as to make a right triangle, sized to be close to the maximum size which your calipers can measure (again, a nice round value here will make the math simpler). While a round pocket is an obvious choice, it is difficult to measure. An expedient option which seems to work well is to spot the features using a V endmill which is more obtuse than the points of one's calipers.
Measure the three holes in a consistent fashion, e.g., left edge to left edge, front edge to front edge. (if you use a V endmill as suggested above, this is simply a matter of poking the caliper tips into the holes to get the tips centered at the bottom). Then compare the measurement to the distance which is expected. Convert it into a ratio by dividing the expected distance by the measured distance, multiply the number of steps by this ratio, and update the number of steps in Grbl with the new value. If the left edge to left edge of two holes which were expected to be 75mm apart measures as 75.03mm, one would calculate:
75mm (desired movement) ÷ 75.03mm (actual movement) = 0.999600159936026 (ratio) × 40 (current steps) = 39.9840064
And then enter $100=39.9840064 into Grbl to calibrate the X-axis. This measurement would then be repeated for each axis.
For the Z-axis, since one cannot do holes on a 3-axis machine, the best thing to do is to mill steps and measure their height.