I'm no pro here but I usually just ue the 75% rule of thumb. If I'm lifting 300lbs, I assume that it's really 225. Either way it doesn;t really matter to me...either I can lift it or I can't. Then the next time I move up, etc. but then again, it's an ego thing when you see yourself throwing up 4-5 plates...arrrrr

Probably 10lbs difference why?

For some reason I think the Smith Machine bar weighs 60 or 65lbs as opposed to the 45lbs of a regular bar.

I think it really all depends on the make of the smith machine and how much extra metal they have attached to the bar. From the 4 or 5 different smiths that I have used I have found them to be around 20 to 25 pounds heavier than a standard bar. I think the only way you would know for sure though is to try and get the specs from the manufacturer of the Smth Machine.

The compensated bar weight on the smith machines at my gym are 15lbs. So subtract 30 from what you would be lifting on a regular bar. It dose depend on the manufaturer of the machine.