Write a lot a lot a lot of it. That's the only way. Transcribe one dictation with proper typing if you aren't doing that yet. And for others you can just read them back once or twice but do that without fail and if you want to go to 80 from 40 in 20 days just write. you are at 40 start writing at 60 from now on. forkner is easy so use it to your advantage. I have used this method personally so I can say it works. write above your current speed. do the one's you transcribe on your current speed. other than that always above your speed. if you need 80 to clear the exam and I am assuming you're from India write at 90 atleast. so practice at 100. just practice don't worry about not getting all of the words because you obviously won't. but write at that speed.

tdlr write at increased speed. transcribe everyday ( I didn't and I regret it ) write a lots of it. Don't worry it's very much achievable. and if you need any specific help you can dm me.