#blocks x 2=_____+ 4=_____
The plus 4 stands for the 1 extra dc and the chain 3 to turn. Crochet in the 5th chain from hook.
To figure the starting chain for a 3 dc filet mesh with an open first block:
#blocks x 2=____+ 4=____
The plus 4 stands for the turning ch 3 and the ch 1 for the space. Crochet in the 6th chain from the hook.
I know this is contrary to many directions that you see on the interwebs. But through trial and error and many, many, MANY test swatches, this formula is what works for me.