Your EVF light meter tells you how many stops you are off the mark. Just change the ISO setting accordingly. F.ex. if the meter says you are about to underexpose 1 2/3 stops, adjust the ISO setting up f.ex. from 800 to 2500 or from 400 to 1250. No need for guessing or switching back and forth to check if you have set it right.
And yes, ISO can be assigned to a function button. You can also assign it to any of the positions on the Q menu. I use both. I like the Q menu to show me the settings I find most important, including ISO. So I can see and change the ISO setting via the Q menu.
However In my opinion there's a distinct advantage to assigning ISO to the front wheel and changing ISO by turning the wheel. Pressing the wheel enables you to choose the ISO value by turning the wheel, but it also enables you to change the settings for the three automatic ISO modes, something you cannot do from the Q menu. I use this to quickly change the maximum ISO value in automatic mode.
I've had my X-T10 for a month and an X-M1 for almost two years. Although in many ways similar (notably the sensor and the lenses of course) the X-T10 is a much more manually oriented camera, which makes it much more fun and powerfull to shoot with.
