Slag
I was never taught, nor have I seen in any publication, a good rule of thumb for speed halfway down the runway. The only thing I have read is that one should pace out a halfway point at home airfield and then on subsequent take-off runs, note the airspeed speed at halfway point in order to build up a practical knowledge of the normal numbers, so that one's brain might notice when this isn't achieved.
If I may quote Aussie Andy, this is my point exactly, although whether that point is halfway or some other convenient landmark
such as a taxi way at a known point doesn't conern me greatly. I would also extend this principle to 'away' airfields, accepting that familiarity will not be as great and thus more conservatism required in the decisions.
Chimbu Chuckles also makes very telling points based on a considerable amount of real world professional experience gained in very challenging conditions, certainly well beyond my personal experiences.
As for the bulldog. The only machine I know which can violate Newtons Laws is the Starship Enterprise. Just because your airspeed indicator says you are going 0knts doesn't mean you are going at 0knts. That is REAL basic stuff, and I give up. I think I will shut up too.
I think you have missed the point here, the 'dog's ASI CANNOT DISPLAY 75% of Vr, because it doesn't read anywhere near that low - what is your problem in understanding that your rule of thumb is unusable in a Bulldog, Period.