I usually use a spirit level on the worktop nearest the cooker, I find if I take the reading there it seems to read ok for sink, the shower and the basin, I tend to make the reading slightly in favour of the shower etc running away completely, it’s a pain the **** If they don’t fully run away, I also have found that the small clear plastic levels that have the width and length level give a reading that is so much off level you might just as well guess at it, if you’ve one of those type next time it reads level check it with a different spirit level, something that’s about a foot long I’ve found is accurate enough.