In a recent Freddie Mac bulletin, they considered 15% instead of 25% gross up. My question is: For Freddie Mac loans I only consider 15% gross up and for the rest of the loans a 25% gross up? thanks for the help

I am not sure that is what the Freddie guidance indicates. You can gross up 15% of the income without further documentation - not that you gross it up by 15%.

For Social Security income (i.e., retirement income, disability benefits, survivor benefits and Supplemental Security Income), the Seller may gross up 15% of the income without obtaining additional documentation. For example, if the Borrower's Social Security income is $1,000/month, the Seller can gross up $150 (i.e., 15% of $1,000) without obtaining documentation that this portion of the income is tax exempt.

Using 25% as the income adjustment factor, the income is calculated as follows:
$150 x 25% = $37.50
$1,000 + $37.50 = $1,037.50
$1,037.50 can be used for qualifying without obtaining tax returns or other documentation evidencing that the income is tax exempt.

The Seller must obtain additional documentation in order to gross up the entire amount of income (i.e., $1,000) for use in qualifying the Borrower.