This amount depends on the previous salary.
For players coming from a fourth year on rookie scale contract, if the previous salary is lower than the MLE, the cap hold is 300% of the salary, and if it's higher than the MLE, the cap hold is 250% (not exceeding the max salary the player can get in re-signing).
Childress 2007/08 salary was 3,631,451. Being lower than the MLE, you have to count 300%, so 10,894,353.
With the same rule, Marvin cap hold for next summer (until he is re-signed) will be 250% of 5,636,142 (5,636,142 being higher than the 5.585M of the MLE), so almost 14.1M (we don't know the max for next year, but it should be higher than 14.1M).
Basically, the cap hold is higher than the real salary to avoid teams signing
too easily other Free Agent first, then re-signing their own F.A.
Hope it helps
EDIT: I made an update on the "Hawks Salaries Information" thread, with estimated salaries for 2008/09 to see what is possible to do. Again, the numbers are estimated.
Once we will know the exact, I will update again and put cap hold for next summer.