Calcualte percentage based on text value in another cell

Try this:

=IF(R5="","",R5/lookup(Q5,{"C","H","S"},{40,40,50}))

I see that you have never marked a thread as solved, so the below is advice on expected forum etiquette. Thank you.

If that takes care of your original question, please choose Thread Tools from the menu link above and mark this thread as SOLVED. You can also access the SOLVED tag by editing the opening post and choosing SOLVED from the drop-down to the left of the title box.

Also, if you have not already done so, remember that you can reward anyone who offered you help towards a solution for your issue by clicking the small star icon (* Add Reputation) located in the lower left corner of the post in which the help was given. By doing so you can add to the reputation(s) of each of those who offered help.

Or:

=IF(R5="","",R5/IF(OR(Q5="C",Q5="H"),40,IF(Q5="S",50,1))

See above about how to sign off your thread.