MrPedantic: Hello RK
Hello, Mr P. I see that you're enjoying a sunny day there in your neck of the woods.
I'm quite comfortable with the sense of completeness in this passage:
I have little doubt that you are, Sir. Let me try to disabuse you of that notion. 
"Of course she was very busy all day long, but whenever she had a little spare time she sat down to spin. Her distaff turned of itself and her spindle span by itself and the flax wound itself off; and however much she might use there was always plenty left."
At the time of speaking, the using and being-plenty-left were complete.
Whatever she used was at least complete, if in fact she used any at all. Really, give it a bit of a think; does this not say that this condition exists at anytime she engages in spinning.
And what of the paraphrases I put for this in my last posting? Did you not consider any of them to be accurate?
For the connection with "may", I would change the passage as follows:
"Of course she is very busy all day long, but whenever she has a little spare time she sits down to spin. Her distaff turns of itself and her spindle spins by itself and the flax winds itself off; and however much she may use, there is always plenty left."
Could 'might' not substitute there for 'may'? Could 'can' substitute for 'may'?
All the best,
MrP
All the best to you too, Sir.