Amount Used to calculate Education credit or Deduction?

We received 1099Q $21,000 and 1098T $14,000.  Our son has room & Board $7000, Computer $1500

How do I put the amount in "Amount Used to calculate Education credit or Deduction"?  TT wants me to try different values between $1,500 and $4000, then recalculate to see what produces the best result.   When we put $4000, we got the best result (more refund) but I am concerned why TT let me put any different values.  How to handle this?