Tool length offset setting - how it works in practice on a knee mill

Back in June (the 18th, in fact), I learned how to use the "tool length offset" setting macro M882 to automatically set up the tool length offsets in the controller's tool table, using an electrical setting device I flashed up. It all went quite well, considering. However, since then I've been at a loss to understand how to repeat the procedure without completely resetting all of the offsets. 

The problem is that the machine uses the (absolute G53 machine coordinates) to determine the tool length offsets, yet every time I change the knee height to move the table up or down, the G53 Z height of the setting device will change and the automatically generated offsets will differ from those already stored. So unless I fancy remeasuring all of my tools each time I alter the table height, the M882 macro isn't going to be much use to me. So much for the tool setting device.

If I'm right, it suggests that the M882 macro was intended for use with bed mills, rather than turret or knee mills. With a bed mill, the tool setting gauge will always be at the same height (in machine coordinates), so this problem won't arise. 

My checking method for the last month or so has been semi manual:

• Using my "tool length offsets spreadsheet", populated with the max/min Z heights and tool lengths for the planned job, I set the quill extension distance between the limits calculated therein.
• Then I specify the current tool to be #13 Martest (Haimer clone) indicator, by issuing G43 T13 H13....
• ...then change to G54 (default work coordinate system) and roughly position the WCS origin Z height to the position defined previously in CAM within a few mm, using the knee height adjustment. 
• At this point, I can precisely set the Z0 coordinate by zeroing the Martest indicator scale and setting Z=0

At this point, the G54 is correctly zeroed for the planned work and the position of the quill will allow for the required range of movement.

For new tools, I've resorted to this process, which basically uses a precision gauge (aka 10.000 mm drill shank), rolled under the tip of each new tool / drill etc:

• Gradually lower the tip using the handwheel, trying to roll the gauge under until it won't fit. Then slowly lift the tool until it just fits. Obviously you need to lower the tool, one step at a time, with the gauge clear of the tool unless you like to chip and break tools.
• When convinced that the tool is at exactly the height of the gauge, go into the "redeem" menu, highlight the active tool (it should know this already from your G43 command), hit enter and fiddle the existing offset value so that the final G54 Z coordinate displayed is 10.000mm. There's almost certainly a simple(r) way to do it but I've been subtracting the current G54 Z value from the required 10.000 value and entering this difference value as the (incremental) tool length offset value. The offset value you need to enter is the difference between the current tool length offset and the required tool length offset.
• If all is well, a few paranoia checks with the planned tools loaded one after the other (both physically and via G43) will end up with each tool 10mm above the Z0 height. You need to issue something like G43 TXX HXX G54 Z10 in MDI, where XX is the tool number.