You cannot easily calculate the memory usage during reload. If there are joins in the script, a lot more memory will be used than otherwise. The best way is to look in the Windows task manager. The only thing that will use extra memory during Save is the compression. Turn the compression off, if memory usage is a problem. (Document Properties > General > Save Format)
The rule of "4 times the qvw size" is just a rough rule of thumb. The real memory usage is sometimes a lot more.
If profiling individual document reloads (for example as part of the final steps of a development track) is ok for you, and you do not plan to repeat this job too often, you could use QV Desktop and Windows Performance Monitor to make a memory profile during reload.
AFAIK (but I'm not 100% sure) the reload process in QV Desktop isn't that much different from the one executed by QVB.exe. If this assumption is reliable, you could even simulate a QVB reload by using the command line options of QlikView Desktop. One of them opens a QVW document, executes the reload script, stores the resulting QVW and exits. Looks a lot like what QVB does...
A command file for generating these profiles would contain things like:
Figure out the document filename
Create a configuration for windows performance monitor
Start Windows performance monitor
Wait for a baseline memory profile
Reload the QVW document
Store the Windows Performance monitor results
Kill the monitor
If you choose the correct export format, you can now open the resulting data in QlikView and calculate a maximum RAM usage level for that particular document..
I admit that I like PowerShell more which would probably be able to do all the steps mentioned in one single run and just spit out a number in the end. YMMV