It doesn't have to be precise.
You'll find that there is a fairly wide range that the plugs will perform satisfactorily.
You shouldn't close them onto the feeler gauge. You should tap them closed gently and check them periodically. A feeler gauge should slide through a gap with just a small amount of friction. If it's tight then the gap should be opened up.
At the start of the thread you said there wasn't an issue with missing under load and now there is.
That seems a bit backwards but whatever.
I would make sure my plug gaps were around 0.7 first of all. Yours would still be 0.9 I suppose?