I would love to see the (Extrusion Multiplier) available on the first layer settings like below:
So.....to increase the amount of plastic being extruded relative to the layer height on the first layer....ie increase the extrusion multiplier....simply reduce the layer thickness below 100%.First Layer Settings (These are applied to where the Bed touches your model. If you have a raft, that means the First Layer Settings will apply to your raft).
First Layer Height: This % will take a % of your Primary Layer Height. If you are printing with a small Primary Layer Height like .1 mm I would recommend a First Layer Height of 250%, to get about a resulting .25 mm first layer height
If your First Layer Height is below 100%, the extrusion amount will remain the same, only the Z-will change, but if you increase above 100%, the extrusion amount will scale accordingly.
First Layer Width: the extrusion width of your first layer, you may find that your first layer sticks better with a thicker extrusion (100%+). I don't have too much experience with this, but I think 125% or 150% would be good starting points.
First Layer Speed: Slows down the First Layer Speed to a percentage of your Default Printing Speed (Other tab)