The range of the IMAX parameter is 0-4500. The default value is 500. The mission planner sometimes does unit conversions - it may change 500.0 to 5.0. It doesn’t do that on the “full parameter list” tab, so take a look there. You may want to increase it if you have an off-balance copter that has a persistent lean.
Of note:
IMAX limits the size of the “trim” value that the copter learns. The unit of IMAX is “centidegrees,” but that is arbitrary - it is a product of the internals of the RC library in ardupilot. 500 means that the integrator can use about 11% throttle differential to trim the copter.
Please check in the full parameter list that the value is indeed 500, then mark this post solved. Thanks.