This is a recreation of the simple ironing test square by VladSalad at Thingverse. It's just a simple 50mm X 50mm X 1.6mm flat cube, I recreated it in FreeCAD and exported the STL so it's not the same file, but it is the same part.

It really helped me nail down ironing settings.

You can check VladSalad's instructions at Thingverse, but after working with this for a bit, here are my tips:

• altering ironing line width will reduce/increase the “comb” effect at the edges of the print where excess material gets pushed to the edges. For a 0.4mm nozzle, try 0.1mm-0.4mm. 0.1mm will reduce this to a minimum but at the cost of 4X extra nozzle movement.
• 100mm/s ironing speed speeds up the ironing process but stresses mechanical components. I have a theory that this also helps by reducing the heat imparted to the printed layers beneath due to shortened contact time. Perhaps 50mm/s ironing speed is a good compromise.
• You'll have to experiment with flow. It's to fill in the tiny gaps in the layer beneath - note the nozzle moves down a bit while ironing so it's plowing the layer underneath. Excess material will be pushed around right off your print. This is not a bad thing since it's easily trimmed away, but you do want to fill those tiny gaps.
• overextrusion/underextrusion in the interior layers of the part will affect the ironing of the top layer - excess material will be pushed around while not enough material will cause gaps. When you adjust part flow, you should also recalibrate ironing using this part.

The photo is of 0.2mm ironing line width, 13% flow, monotonic pattern and 100mm/s ironing speed. Next I tried 0.1mm width and 14% flow, with part flow 100% initial layer and 88% subsequent layers. That part's even smoother in the center but rougher at the edges where excess material got deposited so this photo is just about ideal for my setup. Maybe 0.1mm ironing line width and reduced speed would be better, but flow seems just about right. The second photo shows this is perfect for me (0.1/13%), very very smooth center, just a little combing at the edges and a little excess material pushed off the side. Easily cleaned up!

Print settings:

0.28mm layer height

0.4mm nozzle

No raft, brim or supports

Infill doesn't matter, this part is solid