It would seem that you may have a problem with the collective rigging if you really are experiencing that low an Nr. As suggested above, a more experienced pilot should check the auto-revs and if necessary adjustments made. I concurr with multip.
When trying autorotations, remember that 90% of the benefit comes from practising the entry, subsequent sight picture PFL and Nr control. The landing is unecessarily risky and come the day of a real failure, the most important bit is getting to a safe landing site; you'll almost certainly get away with the engine off landing then.
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Another day in paradise