It is calculated the same on Multilink interfaces as with other interfaces.
The formula for calculating EIGRP METRIC is:
METRIC = 256 * | | (10^7)/(BW) | + | D1 + D2 ....+Dn | |
As you add links or remove them from a Multilink interface the BW on the interface will change to represent the accumulative BW of all links on the Multilink interface.
Hope this helps,
Don