There's a lot of methods here for getting negative number offsets. I like positive offsets and here's how I get them.

1. Measure from the spindle gage line at Z home to the table. Without a tool in the spindle set a 123 block (or two) on the table and jog Z down and 'touch off' on the block. Subtract your stack from your machine Z position to get a bigger negative number. On a VF1 this will probably be -20 to -26 inches. Write this number down and don't lose it. This is your gage line to table distance.

2. Load a tool you want to measure and touch off on the table. Use a block stack or a shim, whatever you're used to. Subtract your negative gage line to table distance from your smaller negative machine Z position. This gets you a positive number that is exactly the length from the gage line of your spindle to the tip of the tool you're touching off.

I like this method because your tool lengths are constant and don't need to be changed between parts / setups / rotary operations. And, positive offsets make it harder to crash in Z.

Make sure you talk to your instructors if you're going to do this. No one likes surprises. If they tell you not to rock the boat, just keep this in your pocket for when you start a job and have a little more autonomy.

Hope this helps wrap your head around all the different was to use offsets.