Focus Test set negative number

“So I did as described in another thread.
For the range, I suggest using your normal focus method as a starting point, then using the “Get Positions” button on the Move window to figure out the Z height that gives you, then make a focus test of Z-5 to Z+5 or Z-10 to Z+10 (for example) to figure out if your existing focus height is ideal or not.”
But I’m not able to set the start to a negative number ( -X ) so I set my focus to the manufacture focus (30mm) and lowered the Z by 2mm setting to 28 mm and ran the test so at 2mm on the test would be the 30mm. Is this correct or am I missing something?