I am not connected to this individual nor do I endorse his product. I am just sharing information.
On another forum I found this interesting GRBL controller using an Android device.
Unfortunately you have to have a bluetooth enabled Arduino and custom GRBL firmware.
Here is his website. It is in Russian but Chrome translates it just fine;
https://sites.google.com/view/arrsoft/программы/android-cnc
I have not tried it yet but watching the video, it looks like you have real time adjustment of parameters.
The upside is that it is not expensive and if you download this weekend, the program is free
You can get app for free from Jun 24, 2017 00:00 GMT to Jun 25, 2017 23:59 GMT.