Alright my math might be a little off here but let's give it a try you want...
Status Variable "Health" = 100 (Current/Full)
Numeric Variable "Cures_Used" = 6%(Health)
And everytime "Health" changes, you want "Cures_Used" to change to reflect an increase equal to 6% of the new value of "Health"
This seems pretty simple math of...
(%Health% / 100) * 6 + "Cures_Used" = "Cures_Used"
Quest doesn't do math all on one line and has to split it up into operations, like you do when you do it in your head basically, well, most people. After thinking about it like that, it's just a simple matter of going into qdk and telling it to do it.
I made two status variables, Health & Cures_Used. When our health status variable changes, it's script tells it to set a new numeric variable called "Cures_Div" with a value of %Health%(whatever our Health variable is equal to) divided by 100, this gives us the value of 1% of our total health. Now we set another new numeric variable called "Cures_Sum" to have a value of %Cures_Div% multiplied by 6, thus setting "Cures_Sum" to 6% of our current health value. Finally we increment our "Cures_Used" status variable by a value of "Cures_Sum" or + 6% of current health.
Realistically, in a scenario where a character repeatedly loses health without gaining any back, this would result in diminishing returns, the "Cures_Used" would always be incremented by less and less unless the character is healed. To counteract this, since I don't know if it's what you were planning for, you would want to do the same math as here but against a characters maximum health to always be guaranteed the same percentage, of course, it would change if the maximum health changed.
For those not in the know about Quest's ability to use simple mathematical functions such as +, -, *, and /, I highly suggest you read the manual. It will start making sense a couple times through, and honestly, isn't very long at all. Here's the code straight from QDK, it should be inserted alongside anyother status variables defined in your define game block.
define variable <Health>
type numeric
value <100>
display <Health: !>
onchange {
set numeric <Cures_Div; %Health% / 100>
set numeric <Cures_Sum; %Cures_Div% * 6>
inc <Cures_Used; %Cures_Sum%>
}
end define
define variable <Cures_Used>
type numeric
value <0>
display <Cures Used: !>
end define
Edit you wanted Cures_Used to increment, not just be equal to 6% of health, ok i'll revise this quickly.