After thining about this quite a bit, I think you are correct that the 8.5 m2 is a little small for good marginal conditions planing on the FF-158.
I also think your 70 cm fin may be a little large to get going really easily.
I need to make sure we are both talking about the same board here.
Here's the specs. I see for the 2004 Free Formula 158:
158 liters volume
231 cm length
100 cm width
73.1 cm OFO width
8.7 Kg. (Wood) weight
9.5 Kg. DRAM
Deep Tuttle fin box
Drake Race 640 OEM fin
Is this your board?
How much do you weigh?
Overall, I think you might be stepping back just a tad too far with your back foot before you get planing and as a result you are popping the nose up slightly.
Just a guess here!
Maybe not, can't really tell unless I could ride your board and rig and our weights are about the same.
If you have the option, I'd try a 66-68 cm fin and a 9.0-9.5 m2 rig.
I think that would really give you the optimum early planing.
Also, how much "under downhauled" are you running your NX3's?
You can get quite a bit more "grunt" from the SW race sails if you run
them a little "fat".
Hope this helps,