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All weight training machines are not created equal. Don't expect the resistance to be the same on all similar strength training exercise machines. One lat pulldown or vertical bench may be actually heavier or lighter than another at the same setting, so you may have to make adjustments to get the workout you expect.
Some Cybex strength training machines number their plates. You can't just add a zero and get the accurate weight--choose the plate that feels right for your workout. When you go to plate loading machines, consider machine weight. The sled on an angled leg press can weigh 50 lb or more, so if you put on 2 plates, it's not the same as putting the pin in 90 on a selectorized machine.
Most Smith weight training machines are counter-balanced, so the bar weighs essentially nothing. If you put the same amount of weight you've been using on the Smith for your squats on a 44-lb free weight Olympic bar, you're in for a surprise.
Finally, remember that even the same brand machines may feel different if they haven't been properly maintained.
Another thing that would be helpful to mention is that some machines, although appearing similar, have slight variations. These variations require different muscles to be exercised, therefore possibly allowing the workout to be easier/harder than expected.
Speaking of sleds, don't forget to take the angle into your calculations. For those who love trigonometry, it's the sine of the angle times the weight. If you and trig don't get along, most leg press sleds are at 45 degrees so multiply the load by .707 to get the real load. For my max sets, I use 12 to 16 plates. That's an awesome 540 to 720 pounds. However, when multiplied by .707, the real numbers are 382 to 509 pounds. The apparent weight of the sled will be on a sticker somewhere on the machine. Don't forget to add it to your total.
Doing free squats is fine but I personally enjoy using the leg press sled. My free squats hover around 315 and I could never load my back with as much weight as I can do on a leg press machine. Plus, the safety stops are a big comfort when I can't find a spotter.
Happy ... and safe ... lifting!