Question regarding clean slate scrolls

Say im scrolling a weapon like so:

1st: success
2nd: success
3rd: fail
4th: success
rest are success

Do I have to use the clean slate scroll right after the failed scroll occured, before using any other scrolls? So if I fail a scroll, is it ok to keep scrolling if I can clean slate the lost slot back no matter how many slots are left?

You can slate it whenever you'd like/are able to.

The game doesn't remember which slot failed.
All it knows is what you can see in the tooltip, which is how many scrolls passed, how many hammers were used, and how many slots are left. And of course it knows how many slots that type of item has when it's clean. So, it can compute how many slots were lost and can be slated back.

ty both very much, this puts me at ease