do I have to use 100ml of liquid ?

Can I use 50 ml  or 25ml  to liquid taper my ativan?

You can use any dilution ratio that you wish.  Most folks just 99:1  (1%) because it simplifies the math.  Actually I usually did 9:1 (10%), sometimes 4:1 (20%)

But remember that the reason we dilute is for greater precision with small dose decrements.  And because ativan is so potent, personally I would opt for more dilution rather than  less.  That isn't so important for V, but it could be important with ativan.