The little scale with the arrows for the micro wheel is that actual hammer spring tension setting-- and that's what matters. The macro wheel just gives you 16 easy larger steps to shift higher or lower from a given set point on the micro adjuster arrows and lines.
Say you have two setups, one with the macro wheel at 16 and the micro scale arrows pointing to 3, and on the other setup you have the macro wheel on 8 but the micro scale arrows are also pointing to 3... that's the same exact hammer spring tension on each setup since the arrows on the micro adjuster are both pointing to 3. However, with the setup with the macro on 16, you can only use the macro to dial the hammer tension down, but on the setup with 8 on the macro and 3 on the micro, you can use the macro wheel to easily dial the power up or down from 3 on the micro scale. It's comes down to how you want to set the rifle up. Personally, my highest power slug tune is on 16 on the macro wheel (with the micro set to whatever the highest power tune needs) and when I want to dial down for lighter pellets I only dial down the macro wheel and never touch the micro wheel.
As far as the micro scale, if you look at it carefully the upper lines with the numbers are in 1/2 number increments while the lines on the bottom are offset half way between the upper lines and are 1/4 number increments. So if you have the micro arrows set exactly to 3 and then dial the micro higher to the very next line on the lower row that's 3.25, and if you dial it up more to the very next line on the upper scale that's 3.5. Makes it easy to make a note of settings.
And yes, if you dial too close to the upper or lower end of the micro scale it will lock out some settings on the macro wheel-- this is described in the instructions on page 17. However, macro 16 and micro 4.5 isn't one of the "unavailable" combos... what might be happening in your case is the trigger doesn't have enough sear engagement to hold that high hammer spring tension setting.
