Please have a look to 45.005, chapter 4.1.1.
The BTS shall send a message “MS POWER CONTROL” that contains those “power control level”.
The algorithm you decide to use in your BSC is up to you: 3GPP didn’t define anything : forget about annex B.2 ! It’s misleading.
Invent your own formula, based on UL_RXLEV and some thresholds.