PlayGamesPlatform.ReportScore adds progress as a percentage of steps

Since PlayGamesPlatform doesn't include a working LoadAchievements,
reporting progress as a percentage of steps allows ReportProgess() to
calculate the proper number of steps to increment incremental
achievements. 

This makes ReportProgress() cross platform happy because using the
Social API on platforms with working LoadAchievements(), progress
can be reported as a total percentage by adding in the current percentage
with the percentage reported by LoadAchievements.  Using PlayGamePlatform
the current percentage is always zero and steps are calculated automatically.

Is there a better way to report achievements using only ISocalPlatform calls?