You can fill a chemical floater all the way up. You'll want to using the lowest possible opening on the floater and monitor your bromine and pH/TA levels very closely. Bromine tablets are acidic and your pH and TA will decrease over time. If your Br levels get too high, you'll need to remove the floater (I suggest keeping it in a 5 gal plastic buck OUTSIDE of the home as the fumes from wet bromine tablets can be very irritating). Do note that bromine tablets add DMH (dimethylhydantoin) to your water and DMH can cause the bromine to become less effective as the DMH concentration increases (very similar to how CYA over stabilizes chlorinated pool water). If the DMH gets too high, your tub can quickly become unsanitary.