Buy yourself one of those tailor measuring tapes and you won't be mismeasuring anything anymore. Get a five foot one and it makes measuring string really easy as you pull it out of the reel or whatnot. If you're doing a two-piece and you're measuring a single pack of string 40' for some reason... why? Take the string out of the pack and uncoil it. Then simply put the two ends together and work your way back to the other side of the loop you just created. Cut there... now you have (2) 20' lengths of string. Do the same thing to find the middle of the length you use for just your mains. Loop it through the racket on the starting point and while holding the ends of the string, walk away from the stringer. When you get to the point where the string is all pulled through and you're holding two ends evenly in your hand, you're done... you've found the middle.
Use these simple technique and you won't have an issue with mismeasuring your string again.
As for your grommet fishing issue... instead of pulling the string through, push it. Cut it at an extreme angle so you create a very nice sharp edge that looks like a sirenge needle and then push it through the grommet little by little by helping it with needle nose pliers. Of course, this works best with stiff synthetic guts and ploy string, not so much with multies that are super soft because they'll bend no matter what.
I do this all the time and I've NEVER had to make a grommet bigger or any other BS to string a racquet. Good luck.