I don't have the Lakemaster chip but in reading the 998's owners manual, it looks like that's the max default water level offset setting available? Guess they need to consider lakes in this part of the country with their dramatic water level changes……..
Looking at the 998's online owners manual pages 107-108:
Water Level Offset
(optional-purchase LakeMaster® charts only)
Settings: -30 to +30 feet, -5 to +5 fathoms, -10 to +10 meters (International Models only); Default = 0
Water Level Offset allows you to change the level of the water being read from the control head. The displayed numbers on the Contour Lines will adjust from the Water Level Offset setting, and the water level offset will be highlighted in dark brown.
For example, if the lake is down 5 feet, set the Water Level Offset setting to -5.
Again, I haven't played with their chip but thinking out loud here - if you were to set the max water level offset to -30 which should change the lake map contour lines to brown 30 ft down. Then possibly try using the Shallow Water Highlight feature to -30 which should change the contour lines red next to the brown to make up the difference to the current water level? In theory, using these two features you might be able to "adjust" your lake map to show a 60 ft drop in the lakes water level. Another possibility would be to try use the Depth Highlight feature instead, which is the green shaded area in the pic below. Just shooting from the hip here so you'll have to be the on-lake tester.