To make the most of the parametric control features of Paragraphic you will often need to use custom mathematical expressions to establish relationships between different values or automatically calculate values based on something that you control.
You can do this by using the Expression input node, which you can add by right-clicking any value parameter that allows input. In the context menu pick Add input node and then Expression. This will add a new node panel above the parameter where you can configure the expression that will calculate the value for this parameter.

In the expression field you can write the expression or formula you want to use for the calculation, and you can add additional variables that you want to use. Any such variables will be added as input parameters above the expression, like size and margin in the example above.
Here’s a video of the whole process for a simple use case where we want to add an expression that controls the spacing of some circles in a repeater to be based on the size of the circles and a constant margin we can set separately.
Like in this case a typical use case for using Expression input nodes can be when you want to reuse a parameter or value that you connect from somewhere else, but want to modify the value in some way. Maybe just adding a margin like above, multiplying or dividing it with some factor or something more complex. Beside basic mathematical operators like +-*/ the expression fields also support using common math functions such as cos/sin/tan/sqrt etc.