Resource Model Question- Daily Rates

The sheet is structured as follows

Column A = List representing salary bands or N/A
Column B = vlookup salary band and daily cost from the assumption tab

Column C= essentially would be populated if no salary band is selected (this would refer to external contractors). It would have to be manually populated.
Column D = (ColumnB<0)*1.13 + (ColumnC<0)*1.13


Is there a more user friendly way to make it more understandable to the user of the spread sheet. I was thinking of conditional formatting. Ideally, what I am trying to show is if there is N/A salary band then the user would be required to enter a rate in column C.

conditional format sounds like the way to go
use ISERROR()

or you could also add an IF statement in column c

IF( iserror(B2), "Please Fill In this Section", B2)
and have conditional format to bold and change to red
but i dont know how C is populated if there is no error

maybe a sample spreadsheet would help