From 100-150 fullness you have max stamina of 100. When fullness is 99 max stamina is 99. When fullness is 98 max stamina is 98. And so on.
The stamina bar is your fullness bar when it actually matters and there really is zero detrimental effects if it dips below 100 so that if actually does drop to 75 when the orange icon appears you can eat to get it back to maximum again and if you didn’t notice the lower amount of available stamina anyway then there really was no harm in letting it drop that far.
When I look at the blue stamina bar I think of it as my hunger and that is why I never need to check any other screen because it literally is my hunger.
If they put a hunger bar on the screen it would literally match and be redundant to the max stamina bar for the values of 0 - 100.
I understood the need and desire for some to mod in the hunger bar before we got this new system because the only way to track it was through the pop outs but now hunger is literally on the screen at all times.
Note: the above was for start of game values. If your max stamina is later 120 then from 120-170 you are at max stamina and then when it drops to 119 your fullness is also 119 etc.