#context par.leading
will give you the current value of leading
at that exact location in the document. However, anything with context
returns content so you can’t use this as a numerical value like 5 * #context par.leading
.
Rather, any calculations and their resulting content should be done within this context
expression, like so (see Why is the value I receive from context always content?):
#context {
let leading = par.leading
// your place calculation
// return your place + box
}