How to define Service Level Objectives without historical data? Assuming I can define Service Level Indicators for a user journey are there any structured methods for setting a Service Level Objective within that SLA without any historical data to indicate what is realistic or feasible? 

For Availability we can often turn to various mathematical approaches that enable us to understand the maximum theoretical availability of the system, although even this is tied back to historical data in the form of Mean Time between Failure (MTBF) and Mean Time to Recover (MTTR).
Without much data you are really guessing, the general thought I have heard before is to start at 80% SLO then work your way up as you have data to back up your goal. There really is not set way of figuring out a SLO without some data. Even taking and using a week's data would be good enough to give you a reasonable idea.