Cadence: Belt drives + Nexus 11 hubs
A Gates drive belt uses an 11mm pitch between teeth.  A conventional bike chain is 1/2 inch or about 12.7mm.  

The upshot is that for the same tooth count, a drive belt component will be slightly smaller than the comparable roller-chain unit with the same tooth count.

Good news - since the front chainring and rear cog use the same size, then the ratio is all that matters, and you can use all the normal formulae to figure out cadences.

The Shimano Alfine 11 has these ratios:

Ratio   0.527   0.681   0.770   0.878   0.995   1.134   1.292   1.462   1.667   1.888   2.153
Step        29.2%   13.1%   14.0%   13.3%   14.0%   13.9%   13.2%   14.0%   13.3%   14.0%    


So assuming your sprocket is 24 tooth that equates to an 11 speed cassette with these tooth counts:

12.6, 16.3, 18.5, 21.1, 23.9, 27.2, 31.0, 35.0, 40.0, 45.3, 51.7


I couldn't find a nice calculator that deals with non-whole teeth, so I've rounded to these numbers instead.  Loss of precision detected.

13, 16, 19, 21, 24, 27, 31, 35, 40, 45, 52


I've also assumed a 28mm tyre with a 700c wheelsize.

Poke these numbers into http://www.bikecalc.com/speed_at_cadence  and you'll get this table:



If you put the same info into http://www.bikecalc.com/cadence_at_speed then you get this table:



ANSWER: on a 50:24 gear combination, with an 11 speed Shimano Alfine hub and 28-622 tyres on a 700c roadwheel, you will be doing about:


30 km/h in gear 9 with a cadence of 89
30 km/h in gear 8 with a cadence of 99