Overriding Default Control Options
In your Volume Mapping, change ‘Control’ from Default to Custom. This displays a number of extra options below which are used to override the default Control Type for the assigned 'Controller Input’. ‘Invert’ reverses the direction that the Volume in Live moves, when you turn the knob. ‘Takeover Mode’ works very similar to Live’s own Takeover mode, the biggest difference being that you can set it on a per Mapping basis rather than only the entire Session.
Rather than using an Encoder, we want to use a button to move the Volume up. Change the selected ‘Controller Input’ option to ‘Button 1’. For ‘Control Type’ change it to Increment. Make sure ‘On’ and ‘Off’ match your Button’s On & Off values. With Switch Type: Momentary: the mapping fires when the ‘On’ value is received. Toggle: the mapping fires when both ‘On’ and ‘Off’ Values are received. Delay: the mapping only fires when the time between receiving the ‘On’ and ‘Off’ value is greater than the delay amount you set (i.e. set it to 0.5 for a half second delay). This is useful for having a secondary function on a button which only fires after a long button press. And finally for steps, set this to the amount of button presses it should take to get from Minimum to Maximum.
As we don’t want the Undo and Volume mappings to fire at the same time, delete the ‘Undo’ mapping which we created earlier clicking on it's 'Delete' Button.
Install your Script, and test it in Live. Remember, the Volume control will only move up if it is below your Maximum setting. There are essentially no restrictions with overrides, you could for example use an Absolute Control on your MIDI Controller but set the Script to treat it like a Relative control so that mappings only fire when 1 of 2 selected Velocity Values are sent from your Physical Input.