How to indicate that a unit is dimensionless

Conventionally we use $1$ for dimensionless quantities, although it may cause some confusions. In additon,

The International Committee for Weights and Measures contemplated defining the unit of 1 as the 'uno', but the idea was dropped.

--https://en.wikipedia.org/wiki/Dimensionless_quantity

Given the way that you've presented your table, I would personally put a "-" rather than a 1 in the units column. This to me would signify that units such as "g, km, s, A" etc. do not apply here. In terms of your symbols, in many branches of physics it is common to use a "hat", "tilde" or "star" notation above a symbol to indicate that it is a dimensionless quantity. For example, if $x$ is the distance travelled and $X$ is the total journey distance, then the dimensionless distance (fraction of the journey already undertaken) could be written as $\hat{x} = x/X$.

If you're interested in dimensional analysis, the you could have additional columns for the fundamental dimensions (Mass, M, Length, L, Time, T, Electric Charge, Q etc.) and indicate the exponent of each dimension with the relevant value, e.g.

symbol description units M L T
x      distance    km    0 1 0
F      force       kN    1 1 -2
Re     Reynolds No -     0 0 0

Yes. In the unit column, put $1$.