1.In aus you first calculate your Limit weight and normal Vspeeds from a table or generalised takeoff data.
2. There is a weight decrement to be taken off the field length limit or whatever limit was most limiting on your performance ( climb, V1 mcg, obstacle) in your case it seems the V1 was it.
3.In checking all these you then have a PMC off performance limit weight which with the deecrements applied gives you a buffer in the same way that a derated thrust calculation should give you a buffer ( we use 6 degrees which equates to about 300 metres normal ops. )
4. Practically you understand that the PMC assists the MEC to adjust fuel flow to maintain commanded n1 however as both Pmcs are turned off you will get a slight overboost in thrust as you take off and accelerate down the runway. The overboost combined with the weight based margins make the higher V1 RTO valid without need for further adustment.
Ps I cannot find a limit for PMC off takeoffs on contaminated RW in our OPS but I am not at work.
I think there will be a difference in min V1 because the uk has an allowance for an unfavorable 5 knot xwind and we dont.
You should then find as increase in min v1 ( ours is 2 knots)