The 12 v heating element is not regulated so 11 volts and 13 volts will give different temps so the readings that are taken will be different as well. Fine for the car since it adjust every few seconds and the alt provides the voltage once the engine is running.
its really important that you get the same results every day. Regular flow bench corrects for altitude and pressure. Along with temp and maybe a few other factors.
You can build a pro style flow bench. Here is a link for the best home kit i have seen. But may need more options for larger ports. Most vacums wont pull more than 200 cfm. With this kit and a vacum even the powerful one they recommend is 145 cfm at 28" and 210 cfm at 10". So totally useles for car heads that flow well.
Flow Bench | Digital Flow Bench | Flow Bench Kit