Flow vs. Spacing in Photoshop

OK, it doesn't seem to do much with flow set to 10%, but flow works OK with lower values.

This is 1% flow and 1% spacing

Do you really need 1% spacing?  I generally use 5% and get perfectly smooth lines.

I tried increasing spacing to 5% and doubled Count to Two which double the number of brush stamps giving you a more less 1% spacing equivalent, but it killed the 10% flow again.  I think you options are to carry on with a 1% spacing, and drop flow to 3% or lower, or make do with a 5% spacing at which flow values of 7% and lower seem to be effective.