PF Raise Rule?

[Snarf]

Classic:
"3xBB + 2x limpers or raisers total=proper raise size."

Lately I've been trying to use this - but I've often found myself doing the math and then saying to myself, "what the %#%@#$%?" and then ignoring that formulas results.

Does that formula assume antes are in play? That would make sesne to me.

Are pot-zized or slightly larger than pot sized raises PF too small?

Example:
Blinds 50 100 - no ante.
On button with some holding I want to raise with.
Three limpers in front.
Pot = 450.
I should raise too....
3 x 100 = 300. Three limpers for 300 x 2 = 600.
300 + 600 = 900.

This formula cranked out at twice the pot?

I guess it would price out people trying to over-call around the back of the table....

What am I missing?