How are Jutsu like reaper death seal tested?

[Professor Sarutobi]

thats true. then i dont see why minato shud have used it on the nine tails. because a lot of people believe that the FTG doesnt use chakra, so if this is true then the only chakra he used was a rasengan. meaning he pretty much didnt use much chakra then he could of been able to seal the nine tails creating a clone coz kushina was the one doing all the work by chaining it down. if you get what i mean?

i believe fuuinjutsu can be created without practicing it, unlike ninjutsu.

well he didn't only use FTG and rasengan, he summoned gamabunta (more chakra for bigger summons and bunta is the boss level summon so a lot of chakra there) and teleported a huge TBB far enough that the village wasn't hit by it im sure that took a lot as well. not sure if the seal he used on "Tobi" would have taken much though.

and yeah probably can be created with theory and prior knowledge of fuuin but you can't master anything without some practice imo.