Here are two popular fin size calculators that you can try:
(I've seen you've tried that already)
When you've tried them, now it's best to forget them ;-)
No, I'm serious. They give you just the guidelines and using the right fin will come from experience. Similar to recommended sail ranges for different boards, they will rarely work as quoted.
Now on to your gear. As an early intermediate it's OK for you to try a 6.5 on your F144, but that sail is way too small for that board - 80cm wide (while an 8.5 is close to ideal). Look at experienced sailors in your area, they will be probably on 95-115 l boards when it's blowing enough for 6.5. To extend your board's range into higher wind a bit, try a fin around 42 (with a 7.5). After a while you'll probably realize you don't wan't to use your Futura with a smaller than 7.5. So take quoted ranges (both sails and fins) with a pinch of salt and try a lot of gear.