Dave,
Remember the THC Up and Dn Acceleration is near infinate (it is shunted / slapped) to whatever your percentage of rapid velocity you have set for the THC
If you look at post #3 on page one...
You listed your maximum (rapid velocity as 1500mm/min) and the THC setting as ~20 to ~30 %...
Say its 20%.
this means your THC slap and go velocity is 300mm/min (5mm/sec) .....
Given acceleration = (Velocity initial - Velocity final ) / Time
In post 6 you gave the steps / mm as 500
hence the time to accelerate from 0 to 5mm/ sec = 1/500 of a second...
You acceleration is 2500mm/s/sec.... for THC
(providing your z axis does not miss a step or two in the process (note 1 step is 1/500 of a mm..... if you can measure that I'll be impressed .... but if it was to miss a step or few the error would be cumulative.... hence if you had a way to toggle the z axis up and down to match the THC UP and THC DN signals.... you shunted it up and down 1000 times (and it missed a step on every cycle) you would accumulate an error of 2mm... which can be a little more visible
But I've read that steppers don't tend to loose one step at a time but the step loss tends to be greater or the motor just stalls.
(can't find the post now, but it was on here regarding encoders, closed loop and steppers missing steps about 5 years ago)
Also you will note that the 2500mm/s/sec acceleration correlates with your Mach3 target smooth curve acceleration nearly of 3000mm/s/sec..
Hence you could probably up the THC % or rapid speed to ~24% (if your rapid speed is still the same as post #3 at 1500mm/min)...... however at 3000mm/min.... your THC speed setting will be at 12%
Try it see if your THC works at 15% / 20% etc if your rapid speed is 3000mm/min.... but according to the calc above it may not.
______________________________________________________
If you had an arduino ..... you could always programme it to toggle two outputs and interface it with two pins on your motion controller, swap your THC pins around and try it.... (I use an ESS so have a load of pins but dod thing about this as a potential THC UP and THC DN testing philosophy)