Probably the quickest is to use the "dy" function:
(cmn (setf s1 (staff treble (meter 6 8) a4 e a4 e a4 e a4 q.))
(staff (tied-to s1) (d4 q stem-down) (eighth-rest (dy -.75)) (d4 q. stem-down sharp)))
The argument is in terms of staff = 1.0.