Does this histogram look right? R10

[albionshire]

I've been advised that a good exposure for histograms should be in the middle, what do you think of this? Is it over exposed or underexposed? My issue is if I get the big curve to go in the middle of the picture, it looks darker, the other issue is I'm using a calibrate colour chart and their suggestion is the 4th grey patch should be RGB 128, to get it to 128 there's no way I can get the curve to be in the middle, am I missing something? This current histogram gets me close enough to 128 so does that mean the exposure is correct? 

[rs-eos]

If one of these replies has solved the original question, please mark it as the solution.  That helps other forum users know which topics have been solved as well as helping others with similar questions. Thank you.  

[albionshire]

They all did, to be fair!