Hello everybody!

I hope there are some guys who can help me!
I'm using the Anet A8 printer. I made some modifications, so I am allowed to print with liquids.
I want to print in a predetermined velocity (that means microliter/minute). I already know how to convert the input of mm into microliter.
But my problem is: when I'm printing a dot (no movement of X and Y) there is another extruder-velocity than when I'm printing a line.
So my question is: How can I calculate the F-Command? Or what influences this command?

Thank you so much for helping me!!!