Yes, thats it

By the way you "KNOW" your FG by lettig a small sample ferment faster at higher temperature.

Back to topic:

Made my new bag and i´m just brewing with it. After removing the bag from the kettle, i got 18L with about 10.2 °P (thats 1.041) - would be a 73% efficiency into boil. Thats better than before
(But i dont trust my measurements yet)
Made a 90 minute mash at 66-67 °C which i had to correct at 60 minutes when temperature fall down to 65 °C. Also made the mash out at 75-76 °C. Controlled actual temperature with 3 devices - well, only 2 devices, one isnt working well.

Still going to make a better bag, my sewing skills suck, so i´ll cut the right peaces and let it sew with my mothers sewingmaschine. Right now i love this brewing method. (But sewing the bag by hand with a sewing needle was a big pain in the ass :D )

Next step is: Im going to build something that i can use my already existing motorized stirrer and USB-Temperaturecontrol via comupter.