The concentrate ended up in the wrong bottle
Updated
On this page8
The unlabeled mixing bottle
Diluted product in an unmarked container in a garage is a hazard communication problem and a usability one. If you supply the dilution bottle, label it; if you do not, say plainly what the customer should write on theirs.
The guessed ratio
An instruction expressed as a fraction of a bottle, or one that requires arithmetic, will be approximated. Fill lines, whole-cap doses and sachets sized to the bottle remove the guess, and they are cheap.
Neat use because you suggested it
A line about using it undiluted on stubborn marks is exactly what the dilution provision excludes from the minimum recommended dilution. It is a sentence with a regulatory consequence, written by somebody trying to be helpful.
Stronger is not better, and customers assume it is
Over-concentrated product can streak, leave residue, or damage trim and coatings. Saying what the dilution is for, not just what it is, gets better compliance from people than an instruction alone.
Which water, and does it matter
Hard water affects some formulations noticeably. If it matters, say so; if it does not, say that too, because an uncertain customer either over-concentrates or does not buy the refill again.
Keep the instruction with the formula record
If the compliance position depends on the dilution, then the formula and the instruction are one document. Changing either without the other is how a position quietly stops being true.
Write the instruction for the garage, not the file
It will be read once, in poor light, by somebody holding a bottle. Short sentences, a whole-cap dose and a fill line survive that; a paragraph with a ratio in it does not, and every approximation it invites is one your compliance position depends on.