I have a problem with this case I am trying to print.
As you can see there is a gap in the wall, there is nothing between these two lines und this gap goes all around the box and for the full height minus the set amount of top and bottom layers.
This makes the wall a tad bit wobbly.
I believe that wall is 2mm thick and my extrusion width is set to .4 mm for a .4 diameter nozzle.
If I set the Shells to 1 that space gets filled with Infill.
If I set the Shells to any number higher than 1 I just get 4 lines and empty space that looks wider than a single line.
If I set the Shells to higher than 1 and Infill smaller than 100% I just get 4 lines and empty space, yes, I tried 99%.
If I set Infill to 100% the wall sudenly has 5 lines and is completely filled.
Why isn't that wall just setup to have as many lines as fit when I set the amount of Shells high enough?
When I reduce the extrusion width to 0.34 and setup 3 Shells I get 6 lines in the preview.
Reduced to 2 Shells I get 4 lines and the 25% Infill I setup and no gap.
Extrusion width 0.38, 2 Shells, 25% Infill - 4 lines, Infill, no gap.
Extrusion width 0.4, 2 Shells, 25% Infill - 4 lines, no Infill, gap.
Extrusion width 0.4, 3 Shells, 25% Infill - 4 lines, no Infill, gap.
Extrusion width 0.42, 2 Shells, 25% Infill - 4 lines, no Infill, gap.
Extrusion width 0.5, 2 Shells, 25% Infill - 4 lines, no Infill, no gap.
Okay, my nozzle has 0.4mm diameter so I need the extrusion width to be 0.4mm.
Effektively it should be a bit higher than that but this should be calculated by Ideamaker with the height of the layer.
So far I tried it with 15% Infill overlap, lets vary that, extrusion width is 0.4, 3 Shells, 25% Infill:
15% overlap - 4 lines, no Infill, gap
0% overlap - 4 lines, no Infill, gap
100% overlap - 4 lines, no Infill, gap
Okay, no difference there.
With 3 shells and an extrusion width of 0.39mm I can make Ideamaker lay down 5 lines for that 2mm wall.
But I am afraid that this kind of tricking the math into producing the result I desire will not change the physics of my nozzle.

The result is most likely that the lines are placed to close together producing an effect like over-extrusion.
Eidt, I just created a test file with tinkercad:
And it shows the same behaviour.