There is no adjusting the recoil reducer.
"the Browning recoil reducer will allow you to shoot more with greater comfort than ever before thought possible.It reduces recoil to your shoulder by splitting the single large jolt of the normal recoil into two smaller impulses,reducing the peak recoil by up to 45%.The internal mechanism has no hydraulic or pneumatic parts to leak or break.The entire mechanism is lubricated at the factory with an advanced silicone based grease.No further lubrication should ever be necessary under normal conditions."
That's from the Browning manual.
Having said that, if you loosen the set screw in the recoil plate and screw off the recoil pad.you'll see a large slotted screw which has a habit of backing out.It's under tension by a large spring which you'll see.
Screw it back in until resistance is felt. I've placed blue locktite on the threads and that seems to keep it in place.
Some owners have a problem with the internal mechanism spring,etc.breaking in time.
There are after market reducers specifically made for the BT99 Plus.