Well, i want to share some experience regarding this. I followed a guide that had the 4 width min rule and other strict rules, but I found it its not necessary.
I made sure I always got to the most noticeable shaking grass every time. Once you found the pkmn you want, start chaining.
I found out going to the shaking grass patch that matches your shaking that is atleast 3 steps form your position works. Any patch that meets your criteria and is 3 or more steps away will work, including those really far ones. However, I didn't test the scenario where 2 shaking grass of the same nature where side to side. I dunno if you go to your closest one, but I went to the farthest one and it didnt broke my chain.
Anyway, I used to follow the 4 width rule thing, and it took me way too long, like 2-3 hrs. Without those rules, I managed to get to 40 chains after 30 mins (can vary depending how much you have to reset)
Also I heard that if the shaking grass happens to be on an edge, if you enter it form the outside (ie: facing the patch of grass) It wont break. So far, I tested it and seems to work.
Thanks for your opinions!
Actually I've been doing that 3 width rule, too. What I think is that your chances of your chain to continue is determined by these rules & how far away is the patch. I did that 3 width rule, & I did manage to get to chain 40 (but I never caught a shiny D: ). But lately, it hasn't been working out for me that rule (maybe bad luck).
Also, the grass happening on the edge IS really true. It's actually a chance of this happening or not. Believe me. I lost the chain by encountering in the edge. I even broke my chain of 32 just by using the PokeRadar at the edge (WHY?! D: ).
Oh well, I guess you have to be a bit lucky if you wanna chain.