the too many arguments error is probably caused because you somehow entered 4 parameters inside an "IF" function instead of 3.
If I understand your question correctly, and need to put an extra condition in each code, try:
=IF(AND(F19="",L19>$M$18),"",IF(F19<3.5,L18*0.01,IF(F19<7.45,L18*0.006,IF(F19<11.1,L18*0.004,"0"))))
=IF(AND(F19="",L19<=$M$18),"",IF(F19<3.5,$M$18*0.01,IF(F19<7.45,$M$18*0.006,IF(F19<11.1,$M$18*0.004,"0"))))
But this seems too easy to fix, I probably misunderstood your requirements.
EDIT: OK, so you need one formula for these two
=IF(L19>$M$18, Code1, IF(L19<=$M$18, Code2, ""))
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