You need to calculate the max flux density. You can use the following formula:
N = V/4*f*a*B
where B = Flux Density (Tesla)
V = voltage (V)
f = frequency (Hz)
a = cross sectional area of core(square metres)
N = no of turns
I haven't done much ferrite design but if I remember correctly a flux density of about 0.05 Tesla should do it. I would use this as a starting point and experiment.
For 0.8cm^2 centre core area and 16V and 85kHz, I get about 12 turns.
I would start with 12 turns and make sure that the peak voltage on the secondary (V2 ) is 16V. If this voltage is less than 16V then the transformer is saturating, and you should increase the turns.
If there is no saturation then reduce the turns by 2 and do the same check again. Continue reducing the turns by 2 until you see the peak voltage dropping, at this point you know that it is saturating and you can then increase the turns by about 40% of the last result. When you reduce the turns you should see the output becomming more square, provided the core is not saturating.
Also at 85Khz, remember the skin effect on the wire, although with a pulse transformer this should not be a problem.
Some ferrite cores have an air gap, I'm assuming that your's does not.