According to the manual for my Mark 4 (may be the same) is Cfun 1-3 and you have to choose Register & then APPLY whatever setting you REGISTER (if I'm reading the instruction correctly). I think you can choose more than 1 choice as the high & low limit
Just tried it out & I am reading it correctly. In Cfn 1-3 I have 3 choices, & if I choose Register I can then select the high limit on the first line, the low limit on the second line & then I had to select APPLY to lock them in.
I think it keeps ISO at 200 so the camera highlight tone priority function can play with the dynamic range by manipulating the ISO value below it. If you shoot RAW, you can do it yourself post processing. Having ISO stuck at 200 or higher is too limiting.