\version "2.19.80"

% store the value in an alist
values.top-level = 5.cm

% function to calculate the value for the paper block
#(define (paper-value v)(and (list? v)(number? (car v))(symbol? (cadr v))))
paperValue =
#(define-scheme-function (val)(paper-value)
   (let ((unit (ly:parser-lookup (cadr val))))
     (* unit (car val))
     ))

% test
\paper {
  indent = \paperValue \values.top-level
}

\relative { bes' a c b }