Ron, I'm a little confused by the question. Do you mean what percentage of that 3500 calories you should eat? If so the answer is 'it depends'
There are many variables that will enter into figuring out your RMR (resting metabolic rate) and fitday is using a 'best guess' based on your height and weight - but it has no way of knowing your % of BF or lean mass. This means that two people weighing the same 150 lbs and standing the same 5'5" but one having 20%BF and the other having 32%BF would get the same 'basal' number from fitday. One would probably be lower if measured (the individual with 32% BF) and one would probably be higher (the other).
If you daily expenditure includes activities performed and lifestyle then you're also dealing with guesses on the part of fitday.
That aside you'd want to make sure that you didn't eat too little (if your goal is fat loss) or too much (if your goal is to gain mass). If a deficit of 3500 calories is what you need to burn a lb of fat (it is) then
technically eating 500 calories less than you burn every day, for a week, would result in a 1lb loss. The straight math of it would lead you to believe that eating 1000 calories less every day would lead to a 2 lb loss, and 1500 less would lead to a 3lb loss. This is where it gets tricky; too big a deficit is stressful and can trigger your body to start holding onto things. It's generally agreed that 1000 calories less than what you expend daily is the furthest you should go deficitwise.
So before you can actually get to that number knowing what type of metabolism you have (slow, average, fast) and how much activity you engage in is a must.
For some really detailed and interesting reading on this topic check out the CKD forum and
The Ketogenic Diet by Lyle McDonald.
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