Hi DTLarca
In the event of the let's say (if) scenario you mentioned, most technicians from experience would notice that the required vac would not be reached in time. If a large pump had been used compared to the volume of the system - we would notice it would rise too much during a vac hold test. As nevgee has correctly noted there are many other considerations like depth of oil (not to mention surface tension). Story - I once had a 1600 kW chiller to vac and after realising my pump was too small I purchased the biggest I could. Then it had to run for 24 Hours to run in - in all it took 8 days to reach an acceptable vac with minor leaks taken to account.
Asides from that we technicians have intuition - a sort of touchy feely tool we use like seeing frost/condensation on a part of the system that we would notice then apply heat to that part to speed up the vac process. We do learn as part of our training the calculations you use along with other affecting points not to mention the experience passed on from our seniors. I have also worked on chillers that have burst evap tubes half filling the chiller with water and DX systems which sucked water in directly where other methods of dehydration were used.