The graphical method is valid, but can be quite inaccurate in application, since the line intersections can vary greatly due to slight inaccuracies in line placement or measurememnt. For a cropped delta, you are much better off to calculate it with accepted formulas (yes, a true delta is a trapezoidal wing with zero tip chord, as far as these formulas are concerned). This can easily be done for you by on-line calculators like the MAC/cg calculators at the Palos Verde R/C site:http://www.palosrc.com/index.php?option=com_content&view=article&id=50:cg&catid=41:ic&Itemid=50
Find the 24%-MAC position, and place the c.g. ahead of it a reasonable distance. FWIW, the MAC of a true delta is exactly 1/3 of the half-span out from the root (meaning that the MAC is 2/3 of the root chord). CL planes seem to like static margins well in excess of R/C models, but with the delta you don't have flaps to complicate things; so perhapsstarting at 10%-15% of your delta's MAC would be more than safe.
(Edited for typos and an added comment)
P.s. I'm back for only one afternoon to do the club newsletter; so I may not be able to respond to any comments until the end of the month.