When using a slide rule, keeping track of the number of digits to the left of the decimal point (DLDP) in the result is fairly simple if you know the basic rule:

For multiplication, the DLDP in the result is:

- the sum of the DLDPs of the multiplicands MINUS 1 if the multiplication is done with the slide sticking out to the right of the ruler's body (for example 2.0 x 3.0 = 6.0).

- the sum of the DLDPs of the multiplicands if the multiplication is done with the slide sticking out to the left of the ruler's body (for example 5.0 x 4.0 = 20.0).

There's a similar rule for division, but that's left as an exercise for the student.