Creating Conditional Rule with Calculation

Hello All -
Wondering if this is possible. I have a currency field called System Fee that the user will fill in during record entry, unless the option from the dropdown field Platform equals “option A”, then the amount in System Fee needs to be 10% of the field Ticket Lift. Not sure how to do that formula with the options presented for creating a conditional rule. Any ideas how I might do this?

So, Scenario One:
Platform: Option B
Ticket Lift: $50.00
System Fee: $3.75 (user manually enters this value)

Scenario Two:
Platform: Option A
Ticket Lift: $65.00
System Fee: $6.50 (automatically calculated as 10% of Ticket Lift because Platform = Option A)

Thanks!