The right flowrate for dimensional accuracy is likely not the right flowrate to end up with solid parts where internal lines are properly smushed together. The sides of a 3D printed object aren’t flat, so if you adjust the flow rate so that the bits that stick out the most are exactly where you asked the slicer to put the edges of the wall, you end up with your internal lines just barely touching each other instead of properly bonded. You want to tune your flow rate to get solid parts when you ask for them, set the line width a little wider than your nozzle to give space for material to flow outwards (which happens whether or not you want it to, but things work better if you tell the slicer it’s going to happen), and then when you’ve got a part that really needs the dimensional accuracy to be right, maybe temporarily use Orca Slicer instead to get its precise wall feature.