Since you only have 7.0 m2 and 8.0 m2 rigs (wow, that's a very close quiver spacing) where do you want the additional range?
Go larger than the stock Freeslalom 40 cm fin if you want better early planing with your 8.0 m2, and also tune your sail with a bit less downhaul and outhaul.
Go smaller than the stock Freeslalom 40 cm fin if you want more high wind range out of your 7.0 m2. You may want to tune your 7.0 m2 with lots of downhaul and some additional outhaul.
With only these 2 sails, I'd think the stock 40 cm fin would be pretty good, and getting another fin 4-6 cm larger (or smaller) would increase your range, but you have to decide if you want a 44-46 cm for better light wind performance, or a 34-36 for better high wind performance.
Also, you will have to decide if you want to stay with the free slalom/ freeride type of fins or go with a more vertical slalom fin for the larger or a more curvy and forgiving design for the smaller fin.
There's alot more to selecting fins than simple span (depth) numbers.
Hope this helps,