Focus distance of Chinese laser lens and other quirks

Hi there,

I recently bought a Chinese K40-III laser cutter and I'm still debating with two difficulties.

I don't know the focus distance of the lens, it's a 12 mm lens but when I asked the seller about the focus distance the seller didn't even knew what I was talking about! Is there a simple way to find the focus distance?

I have th machine cutting almost perfectly, the two quirks I have still is that the machine cuts perfectly in a box of 10 cm x 10 cm from home point but beyond that it needs more power to cut through, is there a way to correct this (mirror alignment is perfect and I clean them every day)?
Second is the debris, this might be normal in a machine like this, I built air assist and a honeycomb table but most of the ashes remain in the material, is there a way to optimize this (get rid of the burnt wood/card)?

Thanks in advance

hi there
first make the ramp test ( put a strip of wood across the table then raise the right side about 6mm up then cut a straight line with 25% power and 10 mm speed then look where the line is the thinnest measure on this point the distance from the material to you laser head , that's the focal distance what you always need to hold)
second the airassist stream must blow in the cut.
greeting
waltfl