Yes, I do agree with some sort of number but if you are that desperate to calculate how much xp is needed you can do so fairly easily.
Kill an enemy and see how much that fills your xp bar. Let's say it gave 3388 xp and it filled your xp bar by 1% (using the gold bar). That means 100% is 338800. If you have 45% left to level, then this means you need 45% of 338800 which is 152460. Divide that by 3388 to give how many of that mob you need to kill: about 45.
I think they did not include this because otherwise there will be too many numbers on the screen. Considering the difficulty of experience gain is supposed to increase almost exponentially at every level, the total amount of xp will just keep increasing until there are massive numbers on your screen.
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