Interesting...
Well, you are not knocking off 3 to 5 knots from your calculations. You're simply calling V1 in a timely manner to ensure that you do not initiate the rejected takeoff at a speed above V1. In point of fact, you're respecting V1, not changing, modifying, etc.
Again, the issue is dependent on when the aircraft was certified. Under the old certification, you really don't want to allow the aircraft to attain a speed above V1 before the stopping actions are initiated. If you call V1 at V1...have an engine failure at V1...and you try to stop (assuming you're field-length limited), you've got a problem....because, before you know it, you're going V1 plus a couple...then, you initiate the rejected takeoff drill....BIG PROBLEMS!!!!!
We're talking only a second or two in time.....makes a big difference for 'stopping'...not much difference in 'going'. According to engineering data, even if you lose an engine at, say, V1 minus 2 or V1 minus 3...and continue the takeoff...you'll find the reduction in screen height is reduced minimally...like maybe ten or so feet. (Still above the 15 foot wet-runway figure)
But, if you lose one at V1 minus 2, and you try to stop...you'd better hope you do things very quickly...you'd better hope the brakes are in good shape (not worn to just above minimum limits)...you'd better hope the runway surface at the other end of the runway is not contaminated...or full of rubber or other crap....in other words....if you reject at V1 (and you're field-length limited), things have to be perfect for you to stop before the end of the runway.
In contrast, losing one at V1 minus 2 and continuing the takeoff will have minimal consequences. This is why the manufacturers say, as you approach V1, you need to be more 'go minded' and less 'stop minded'.
PantLoad