Is a black cross on a poo chart much too negative? Am at end of tether!

[MinkyBorage]

Long story short, DD1 3.5 is completely dry, but was witholding and constipated, and has been soiling for nearly a year. She is now on laxitives (movicol) and over the last couple of months has completely turned around, has been pooing regularly on the potty and toilet (which she NEVER did before), so I've eventually bitten the bullet and put her back in knickers. She does a poo in her knickers every couple of days, which is great progress really, but to be honest, I can't help thinking she is being lazy in doing this, it's always when she is distracted, and mainly because she refuses to go when she needs to.
Aaaaaanyway, she has asticker chart for poos which has a picture of a scooter on each day, and if she gets through a day without an accident then she gets to stick a gold star on the scooter, which is a clear enough message. Two weeks of gold stars equals a scooter. However, I'm beginning to feel the need for a black cross through the day when she does soil, but I know this is propbably born out of frustration and desperation, and suspect it's too negative. On the other hand, she needs to know she shouldn't soil.
What do you think?

I know how frustrating it is. My ds took ages to get to the poo in loo stage. However, I found the only thing that worked, and it took a while to sink in, was complete ignoring anytime he pooed anywhere but the loo. I would change him in almost complete silence and just say, nevermind next time we'll do it in the loo.

I had to dig my fingernails into my hands in frustration though!

Also a shorter incentive worked better for us. I think I did something every couple of days.

Thanks for reply, I've tried EVERYTHING!!! Maybe you're right, I could start with a two day clean incentive, and then build up.actually that's a brilliant idea!

I also had a goody bag of tat that he could choose something out of everytime he did a poo in the toilet. That worked wonders! It is soooooo stressful though.