The brake on a lawnmower is on the layshaft not the axels.
As such it locks the diff, which is exactly how the P position works on an auto box.
In this condition each wheel can only turn in opposite directions.
Thus the brake does not stop the wheels from turning so no amount of adjusting will stop the wheels turning when the mower is on a stand.
With the wheels on the ground when the brake is applied if one wheel can only go forward and the other backwards then you come to an abrupt stop regardless of what direction you were traveling in.
Now because the brake & the clutch work on a single pedal, you adjust the brake so that the ROTOR is locked just as the dive belt gets enough slack to slip on the pulley.
There should be an adjustment on the brake rod which is usually sprung as well as the adjustment on the brake caliper itself.
Naturally you do not what the mower to be braking and clutching at the same time.
So first adjust the rod so the brake lever does not move till the belt has enough slack to JUST START TO SLIP / only just drives
next adjust the caliper so that with the pedal in the locked parking position you can just push the mower against the brake, then tighten another 1/2 turn.
Check again that the brake is not still on when the slack is taken out of the drive pulley.
Next find yourself the steepest hill on your property , with the engine running, apply the parking brake.
If it holds, all is well if not tighten the caliper nut in 1/6th turn increment till it does.
You need to have the engine running just in case you need to toss it into reverse to stop you imitating Mulga Bills Bicycle.
These tests need to be done both in the forward & reverse directions.
If you stop so abruptly your filling fly out of your teeth, loosen the spring adjustment on the clutch / brake rod and if the mower rolls before the drive kicks in tighten it a little.
Getting the brake/clutch balance right is a PIA trial & error job but once done can change a pig of a mower into a pleasure to use.
