No, no, no. I'm not sure where you're getting 100%, 200%, 600%. It's 1%, 2%, 6%.
If feed override is LESS THAN 1%, feedhold, set LED.
If feed override is GREATER than 2% AND less than 6% AND the LED is on, do a cycle start.
In practice, you turn down the feed pot (analog pot on PoKeys for me), feed slows, but in Mach without this brain, NEVER stops but creeps slowly. With this brain, when it gets to less than 1%, it feedholds, sets LED. Turn the pot UP, it gets above 2% it cycle starts. The 2% to 6% is there because if it's 2% to 3% (like the OP's feedhold brain) and you turn the pot too quickly, sometimes it misses it for some reason.
And this does work in conjunction with the other brain of MY 2, HOOD_FRO (credit where credit is due, Hood, your name in lights!). That one reads the pot, on MOD: 126 IN, PoKeys pin 45 (I think, it's 43-47), and sets feed override 0-150% on the pot.
This, for me, is essential, having analog pots to adjust Feedrate and Speed, especially feed. I can't live without it!!! Old CNC guy from the days before membrane control panels. I can't grab the mouse and drag no steenkin' screen slider to control feed rate!!
There is one issue with this, for some reason (Mach bug?). If you're in Single Block, doing this gets you OUT of Single Block but the Single Block LED on is still lit on the screen!!! Click Single Block off, back on, it's good, but scary the first time you experience it.
The OP's feedhold brain does something different from mine and therein may lie the MDI line problem. I don't have the problem although I thought I did! Might have something to do with the timer in his. I don't understand enough to know about that.
Hood wrote these in one form or another, I modded it slightly for MY PoKeys pin and separated SRO out to a separate brain, they were combined in this brain originally.