Ok to touch off against the work and set the handwheel dial or note the reading if a fixed dial BUT move cutter away from work before starting.

Your mistake is in starting the machine with the cutter in contact, though also look to ensure the stock is held fully along the available length.

One well-tried method if a centre-finder is not available, uses a slip of thin paper to set the staring-point.

Measure the thickness of the paper, then "glue" it to the face of the metal with a smear of oil, with the cutter a couple of inches back from the centre of the face to be milled (and the machine off!).

Now start the mill, and very carefully advance the table until the cutter just bites the paper.

Lift the cutter clear / lower the table, stop the motor. Move the table so the cutter is now in the area of the start of its travel, which is in fresh air, not on the work-piece.

Advance the table by the paper thickness, and set the dial or DRO to 0. (The former assumes moveable dials – if not, note the reading. I put a pencil mark on it, too.) Now advance the table again, in the same direction, to the intended cut width.

Lower the cutter / raise the table to set the vertical depth appropriately – it may not be the full thickness of the stock.

Lock the travels that need stay constant, turn the machine on and start the cutting. Go gently on bringing the cutter into the metal.

On a light-weight machine especially, the depth and thickness being removed should not exceed about a third of the cutter diameter, which for cutting an open surface should really be an end-mill.

Always cut down-hand, i.e. with the work advancing against the advancing cutting-edges. The other way, "climb-milling", pulls the cutter into the work, and is safe only on very rigid machines with very high inertia and minimal backlash.

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A note on using the vice:

If the work does not extend inwards past the axis of the vice-screw, any slack in the vice can rotate the moving jaw about the inner corner of the work, reducing the grip of the jaw's outer end on the metal. My way round this, is to put a piece of round bar of the same diameter as the work width, vertically in the vice right at the other end of the jaws – and take very careful cuts.

It will not.

You bring the tool into contact with work or fag paper feeding in one direction, back off tool in other direction then put the cut on in the same direction you used to locate the edge so both times you were feeding in the same direction so no backlash issues.