I am kinda new to simulations, and need some guidence.  I have a spool valve for a diesel application that we are trying to manufacture completely from CNC's, now that being said instead of tenth's for tolerance we have thousandths. I have been asked before any investment in trials is to begin, they would like to have a simulation run to see how much leakage the customer could expect.  I have a known upstream pressure of 18 bar and the downstream dumps to atmospheric, with about max condition of 5 thousandths of clearance between spool and housing in a .375" ID.  I have run other simulations before using a known flow at pressure but never with two known pressures to find a leak value.  Any suggestions????