The percent could be a bit higher, maybe 22%, but it works exactly how it sounds. Don't d.i. the grab, and time the shine for right when he tries to re-grab you.
If the Marth knows how to correctly pivot grab, he can avoid the shine and grab you anyways. Trying to shine out at the correct percents is useful and helps you weed out the knowledged Marths from the ones that just spam grab and hope it works.
If you find yourself getting perfect CGed by a pivot grabbing, combo heavy Marth - focus less on d.i.ing the throw and more on d.i.ing the hits once you get past the general CG percentages. At about 30% and higher with neutral d.i., you should be able to jump out. This is why Marths start the utilt regrabs at this percent. Try to d.i. the utilt down and away (so it would be in the direction the sword is swinging and not the way Marth is facing), and maybe you can escape without taking too much damage. If you survival d.i. it you go straight up...and thats never good.
Remember that on FD you can't do much to avoid CGs altogether, so getting grabbed more than 3 times is going to cost you the game vs. a good Marth.
