You may want to check out
Rusty Moore's Visual Impact Cardio for a fairly extensive treatment of the subject. The short version is this: you aren't going to lose muscle if you are training with weights and doing a reasonable calorie restriction.
Keep in mind fat loss is not always linear. In other words, you may find that you plateau for a week or two then suddenly drop a pound. It has to do with the way fat cells shrink, etc.
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