You can direct the pressure and limit the powder to under a particular area of the stone, I've done that. I've also seen people on YT dump a table spoon worth of powder on the plate.
You want a uniform layer of abrasive between the stone and the plate, having a mound of grit is a bad practice.
A small amount, like half a tea spoon is good enough.
Rafael, you made three important statements (underlined) in your post on which many people may not have figured.
Many may think lapping is just going back and forth over an abrasive material.
Sometimes one has to work just a small area at a time.
Like a plane sole, if it is humped in the middle, work it on a thin strip of abrasive so only the bump is abraded. It might need to be worked sideways in some cases.
jtk
"A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty."
- Sir Winston Churchill (1874-1965)