So....if your first layer is printing thicker than you think it should be printing...and lets confirm that your first layer is printing 0.1mm thicker than it should, irrespective of what the setting for the first layer height is, then you need to lower the mechanical position of the nozzle in relation to the print bed.
This can be done the easy way....or the hard way.
The easy way is to adjust the Global-Z setting value in a negative direction. Note that the setting can go negative!! Adjust by a small amount and do a test print...then adjust in small steps until the layer is at the correct height.
The hard way depends on your printer and then method you use to level the bed. If manually, use a thinner piece of paper. If auto, refer to the manual.
And btw...the first layer height setting is to allow you to adjust the height of the first layer ....not to SET the first layer height And yes, there's a difference!
And a second btw...the First Layer Width setting in S3D is somewhat redundant. Adjusting it simply make both the extrusions and any gaps between them thinner or wider! Its only real use is to adjust the adhesion between raft and part when using a single layer raft.