From Internal Employee to Consultant

I’m currently the systems administrator for several offices for my employer. I’m going on maternity leave soon and want to come back on a consultancy basis. They are happy with this solution, not wanting to replace me, but I need to change my rates from my current salary to an hourly rate. I know it needs to go up, but is there a standard (or semi-standard) percentage above which a consultant can charge?

Thanks for any help you can give.