There is usually a maximum allowable flex overall, expressed either as an N1 or % or temp value. This is regardless of environmental factors or weight.
Well as you would no doubt know the effect of runway length on the V1 Vr relationship then you would see which is limiting.
I fail to see how comments like
Incorrect v speeds and thrust settings would make things difficult especially with an engine failure
can go left un challenged, as without an engine failure what is going to be "difficult"?