gistfile1.txt

[samth@huor:~/sw/plt (master) plt] r ~/tmp/ptr.rkt 
diff = -104
x = 7f6f077a0fd0, *x = 7
y = 7f6f077a1038, *y = 5
p = 7f6f077a0fd0, *p = 7

ptr.rkt

#lang racket
(require ffi/unsafe)

(define (addrof v) (cast v _racket _intptr))

(define x (box 3))
(define y (box 5))

(define xi (cast x _racket _intptr))
(define yi (cast y _racket _intptr))

(define diff (- xi yi))

(printf "diff = ~s\n" diff)

(define p (cast (+ yi diff) _intptr _racket))

(set-box! p 7)
(printf "x = ~x, *x = ~s\n" (addrof x) (unbox x))
(printf "y = ~x, *y = ~s\n" (addrof y) (unbox y))
(printf "p = ~x, *p = ~s\n" (addrof p) (unbox p))

Note that if we replace diff with -104 the result is unchanged.