A general question about expending practice points:

When I got to 8th level I expended one practice point to train in String.
The first time I sang a song that used a stringed instrument, my skill
raised one point to 9. That indicates that expending the initial practice
point to get the String skill gave me a skill level of 8.

On the other hand, when I first started my character I expended one practice
point in Sense Heading. The next time I went up a level in Sense Heading,
my skill level was only 2. That indicated one practice point spent and one
skill level gained.

So, does expending a practice point give you a random amount of skill levels
based on your INT and/or WIS score or does it only give you one skill level
per practice point? If the second case is true, how do you account for me
getting 8 skill points in String while only expending 1 practice point?

Reason for asking: I am about to embark on a secondary career as the
Kelethin brewmaster for E'ci. I haven't expended any practice points in
brewing yet. I want to get my skill up to 10 before I start the learning
process. Just need to know if I only need to put a couple of practice
points in it or will I need to expend 10 practice points.

Thanks
Galtin
E'ci