No matter how many miles you have and no matter how much the charge is, you will only be able to redeem by $25/2500 increments. So it makes no difference if you purchase something for $50 or $5000.
I think I explained this upthread, but here's how it works (in my experience):
If you have the exact number of points or more required to wipe away a charge, you can wipe away the entire charge.
If you have fewer than the exact number of points, you can only redeem in 2500 increments.
Example: If you have an $87 charge, and 9000 points, you can wipe away the entire $87 charge. If, however, you only have 8000 points, you can only wipe away $75 worth of the $87 charge (2500 point increments x 3).
Someone asked me how I know -- again, just my personal experience with this card -- perhaps it's worked differently for others.