How much wind?
With the sails you suggest (both have lots of low wind power when correctly rigged) you
can probably expect to be planing in 9 knots (maybe a bit more) and if you can plane, you may be able to go upwind "on the fin" which requires you to tip the board slightly lee rail down, to go upwind.
But you have to get fully planing first. This means you have to sail enough off the wind (beam reach or perhaps slightly lower) enough to get your board fully planing, then get into the straps and all the way back.
Once your board gets up to speed, you lift slightly with your front foot (in the front footstrap) and push across the top of the fin with your back foot (in the rear footstrap, heel down over the upwind rail a bit).
The board should rip upwind very fast on your 70 cm fin.
Hope this helps,