A little thought first
Giving a little thought first will show you the fallacy of any such VM rule as "half of the machines physical memory"-and that's true for *any* direct ratio whether it's half, 2X or 3X or whatever.
First, the total memory required by a PC dependson what it's doing. If you can't predict that then it's foolish to limit that memory by setting a maximum size to the swap file-all that will do is generate 'out of memory' crashes when you need more. Personally I'd rather the PC slow down but keep working vs. crashing.
Second, a direct ratio assumes that a PC with less physical RAM will also need less virtual memory. Huh? Take your 'half' ratio. That says that my PC with 64MB of RAM needs only 32MB of VM while my PC with 256MB of RAM needs 128MB of VM. I'd say it's more likely to be just the opposite.
If you simply *must* apply a ratio then at least make it a reverse one. Say some figure (I like 8192) divided by the amount of physical RAM. That gives the 64MB PC 128MB of VM & the 256MB PC 32MB of VM. How did I get 8192? I determined by trial & error the amount of VM I though appropriate for the 64MB PC & worked backwards to the factor. Then I applied it to the 256MB PC to cross-check that the factor I derived seemed reasonable.
In practice I find it best to put in plenty of RAM & hard disk then let Windows handle the swap file.
