A sense of humor.

Seriously.

Wait, isn't a string faster than pulling out a chart and counting squares?

If anything you should bring the chart so you don't have to worry about what the judge says. Both players should have enough knowledge to figure this out.

A chart is fine and all but I think it can't take into effect hindering and blocking terrain once you're counting over a large area.

Pull out the string, check, verify that both players agree and if there's a dispute, roll off, and most importantly...have fun!

Yeah, I use the midpoint method too. With shortened ranges usually you are only counting 3 squares. Also, most of the time if you find the diagonal everything on the diagonal has LoF, then you can tell which side has LoF and witch side is blocked. I don't carry a LoF tool, but a quick yell of "who has a string?" usually turns one up if needed. Works pretty well, I've never had a player argue with a LoF ruling.

My most important tool is my phone. It has the HC rules app, timer, and pdf files of the player's guide. And if I have reception I can access HCRealms rules forum.

Although I also bring paper copies of the PAC and rulebook and lots of extra stuff like maps, dice, smoke could tokens, etc... Although that's the stuff I cart around in my carrier even if I'm not judging.

One other thing I bring that most judges should include is extra figures or teams. Sometimes we'll have new players show up or players will forget part of their team. Being able to loan figures so they can play keeps players happy and more likely to come back.

Its in the rules and I respect that, but the string is clearly flawed because you can not accurately determine the center of the square. In order to use a string for LoF accurately you would need 5 pieces of string, not just 1 and even then a person who knows nothing of geometry could still make a mistake.

I think that's taking things a little too seriously. We're talking a game here, not higher mathematics. If you can't figure it out with a string, then I don't know what to tell you besides let the judge rule on the string...

The judge should bring matches to matches.

The string/straightedge, coupled with my ability to determine the center of the squares, works well enough for me to make an accurate judgement on the LOF. If a line doesn't work well for you use another method: to the best of my knowledge there's no "LOF Determination Police". I personally use a line or a look at the geometry based on which is quickest in the moment because I have found them to be interchangeably reliable and accurate for me.
The OP's assertion that "the string has to go, string is not how LOF is determined" is demonstrably false. It does in fact work, and it works accurately. I can see you arguing that you prefer a different method, but arguing that the line is intrinsically unreliable and insufficiently accurate is merely opinion and with significant anecdotal evidence to the contrary.