Set range (A)

This was a concept I mocked up while thinking of ways to be able to set a range within a min/max scope, where you might use sliders for fuzzy adjustment (live preview); increment/decrement buttons for fine tuning, or enter the values directly, with the option to enter the difference between the two limits.
I was thinking that if you most recently enter a lower limit the plus sign and the arrow would show as it does here with the upper limit being auto-calculated. And if you set the upper limit more recently then the arrow becomes a minus and the plus becomes a down arrow, with the lower limit being auto-calculated.
(version A is the non-flatty)