Low engine compression, or is it?

[Chuck Schott]

Greetings Listers,

Per repair shop yesterday, the compression on my 4kq is 155psi(40psi
leakdown) on the #1 and 120psi(20psi leakdown) on the rest.

They say I'll need a rebuild "before long" but could not really provide any
elaboration, hoping you can.

Although these values are less than the normal range of 130-174, they are
still above the minimum of 102 psi.  I guess I thought that this would be in
the range for what you would expect after 165k miles, Or is major engine
work really just around the corner?
Is it a given that rings and/or valves would be at fault or could it be my
leaking head gasket?

I'd like to recheck the compression but have a couple of questions if any of
you could help out:

Should I crank until pressure stops going up, or just for a certain number
of cranks?

Can I check "leakdown" with a basic compression gage (pressure release
button on side)?  How long do I wait?

TIA,
Chuck