Do the calculation on the SEG_Count field with the following settings (I assumed that there are no Null values in these fields):
Parser: Python
Code Block: Checked
Pre-Logic Code Block
def SEG_Count_Cap(SEG_Count, Join_Count):
if SEG_Count > Join_Count:
return Join_Count
else:
return SEG_Count
SEG_Count:
SEG_Count_Cap(!SEG_Count!, !Join_Count!)