This first layer is always the hardest to achieve. After the doing many 3D Prints you will get better.
Though this is not something that can be set to a number, it is part learning process of 3D Printing simply speaking it is the Art of 3D Printing.

There is a method that will assist him in this is printing process print on Blue tape.

Now have never used Blue Tape myself, as when it was first suggested to me I was already 3D Printing on Vinyl transfer paper tape.
I have a couple videos attached below to show how easy it is to get the hang of 3D Printing on the Tape Method.

Installing Vinyl Transfer Paper Tape
https://youtu.be/tJG1i2utfB4

Will Vinyl transfer paper Hold my Part
https://youtu.be/qzWKE3TO9UU

I suggest learning 3D Printing on tape because is is more forgiving, meaning you can have larger tolerances and still get good adhesion.
When you get to the point your print has good adhesion and is easily removed from the bed you are ready to try the build surface again.

Will Vinyl Transfer Paper release my Print
https://youtu.be/Y5ut1OtMSp0