I certainly wouldn't waste time memorizing that info. I get better every time I play at 'eye-balling' it, but I wouldn't try to memorize the charge distances.
Anyway, measuring of movement has been hotly contested. In my small group, we don't measure movement. However, some people are of the mind that since you can measure range at any time, you should also be able to measure movement. I'm not a fan of this, as it seems to me that it removes a major skill element to the game. Since Melee is deterministic, and most ranged combat is random (though you can measure the range at any point during your move to remove the chance that you might miss completely.) This means that the ability to move your units skillfully is extremely important. Using a ruler to measure that movement removes that need for skill.
Also, as you stated, that they didn't make the ranges on the game ruler with any indicators of how far any disk might travel. Because of these design choices it seems to me that the creators of the game intended for the skill part of the game to be in the movement phase. It's my opinion that if you take that skill out of the game, you are forcing it to stand on it's weakest element, which are the random damage effects. Since you will always know before starting a move if you can charge or not, you'll never have to take a serious risk with your units, except those you are pressed into by an opponent with strong ranged contingent. I also feel like you would end up playing seriously defensive games, where no one wants to move into range to be charged by their adversary, and since you'd always know what that charge distance was, people could just dance around each other until ranged units pick your armies apart.
Anyway, those are just my opinions. There is no official stance on this from FFG as far as I'm aware. The one rule I can think of is that opponents in a tournament have to agree on a single range ruler for both of them to use during the course of their game. I don't recall if there is anything to say that it must be an official one from FFG, but I wouldn't let my opponent use anything that wasn't. Not in a tournament anyhow. In casual play, I'd let you play how you want, as I'm just happy to get a game in. But if that game counted for anything, there is no way...
As for you and your specific group. It's your game. Do what makes sense to you and what makes the game fun for all involved.