1st Level : 2nd Level Service Engineers Ration

Hi! I would like to solicit your ideas based on your experience that required ration between 1st Level Service Desk Engineers versus 2nd Level Field Service Engineers. My dilemma now is to find out the ideal ratio between my 1st level support versus 2nd level field support. For example, if I’m going to build a team of Service Desk Engineers of around 15 staff, how many Field Engineers do I need to have in order to support my client? Hope you could help me on this?

Thanks.