Another solution :)
(the 2 lines are of course drawn with the Helper lines for have Tangencency ;)


Not so a big deal ;)
ps Else this function exist in Google Sketchup ;)
Smartpushpull ;)
http://forums.sketchucation.com/viewtopic.php?f=323&t=23341

With the Michael's solution you must calculate the height before and the wight of offset ;)