Not sure if I would consider myself even an 'average programmer' but a lot of what I see here would be 'obfuscated'.
Also, since we're talking about someone else reading the code, I didn't see anyone mention comments.
My simple approach might be something like this.
;;
;; setup default values
;;
(if (not (setq start (getint "\nEnter first plot to renumber<1>:")))
;; if user responds with <enter> set default
(setq start 1)
)
(if (not (setq stop (getint "\nEnter last plot to renumber<1000>:")))
;; if user responds with <enter> set default
(setq stop 1000)
)
(if (not (setq stepping (getint "\nEnter amount to increase each number by (negative values decrease)<1>:")))
;; if user responds with <enter> set default
(setq stepping 1)
)