Crocheters often make afghans, and if your afghan is done in one piece, often requires a large number of chains.  It is so easy to lose count while chaining, and sometimes is difficult to go back and count them accurately and get the same number every time!  To help in counting, make use of the coilless safety pin markers (Yes, we have them!) or split lock markers that are available and should definitely be part of your stitching notions bag.  Insert a marker every 20 stitches or so and it will alleviate the need to go all the way back to the beginning of the chain when you lose count.

If you are altering the width of the written pattern and you need to change the number of foundation chains, figure the number of chains you think you will need, and then add a few extras.  Having too many is better than having too few, because you can always unravel the extra chains and just weave in the tail.