Turnaround timing: How much below the lead time it should be?

I have been developping a Matlab Simulink model to a client and it is required the model to be compatible with a time-rate of 1ms. My client will integrate this model with a parent model that runs at this time rate in a multi-core machine.

In spite of this will run a multi-core machine, my model will share a core with other tasks, that means I have more reasons to make sure my model wont consume a lot of processing and won cause task overruns.

I believe that there is a good practice rule or conceptual definition that estipulates a desired turnaround time in depending on lead time. There is?

I wanted to know how much bellow the lead time (1ms) I should worry to keep the turnaround time bellow.

I would appreciate if you could point any reference.