I would say that there is no rule of thumb.
Even if you have historical data, you could come up with 5:1 or 1:5, all depending how your system is used.
One days worth of data will give you some information, and one week will give you a very good estimate.
You can even try the free version of RRR|License, and it will tell you the ratio that is optimum for you. You can use the change planner in the test version, where you can put in a future number of expected users:
Best Regards - Misi, RRR AB, http://rrr.se
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as Misi has mentioned, you can only really come up with a ratio once you have the required data to calculate.
Once you have the data, you can see how long each user spends utilising the license and you can calculate where to re-allocate the licenses for optimal ratio's.
I have an evaluation version available for license management that you can download from my site, it will peform calculations for up to 20 Users and show usage, trending and give you the top users that are candidates for re-allocation of Fixed/Floating licenses allowing you to improve the ratios.