Combining a 2 part formula

Hello,
Here is a screen shot:

What I am trying to do is create a formula for In Stock so that when Condition is New, Quantity decreases by 1 (and if the Condition is not new, it stays with what is in stock). However, as you can tell with Row 3, that is not the case. My formula thus far is
= Quantity & IF({Condition (from Maintenance)} = ‘New’) - 1
What am I missing?

Furthermore, if Condition has ‘New’ twice in the column (or however many is in the Quantity), such as Row 1, in stock should be blank or 0, or if there is nothing in Condition, In stock should equal Quantity.

I have it so when stock gets to 1, the status changes to Order More, and would like it to change to Out of Stock when it is 0
IF({In Stock} = 1, ‘Order More’) & IF({In Stock} = BLANK(), ‘Out of Stock’)

Is this doable or do I need many more columns and filter / group items?