Hi Linda, I agree with Lunacat, it's also the method I use with the football players, I train. You should be able to do some where between 10 to 12 reps with weight, and the 11 ot 13 is just too hard or causes you to go out of proper form. That's the right weight for you. This can even vary from day to day as to how you feel, how good your diet as been, etc. Don't be afraid to go down if you need to, but should always go down, if you feel like your going to lose your form, or have lost your form. As there are good days and there are bad ones, and you can only go with the flow. You'll still get a good workout though even if you have to go down on one of your bad days. So there really isn't anything lost, and the next time you may even be able to go heavier.
Kit