that's weird, because it's no steady acceleration (by decelerating by 10% you mean 10% of the actual speed? then it will never stop).
i assume you mean by 10% of the initial speed. let v0 be the initial speed. since it's a steadily accelerated motion we can use the formula
s = s0 + v0*t + 1/2*a0*t^2
a0 is -v0/10 (i drop the units for now, even though it's not wise to do so in a solution. it would make sense if the factor /10 would have the unit of a time, e.g. s). the sign means antiparallel to the velocity). now we derive the above equation and get
v = v0 + a0*t
which should be zero at the point given, so
0 = v0 + a0*t or t = -v0/a0 = v0/(v0/10) = 10 (note the sign here!)
which we use in our first equation:
s = s0 + 10v0 + 50a0 = s0 + 10v0 - 50*(v0/10) = s0 + 10v0 - 5v0 = s0 + 5v0
now we set s = 1024 (the point it should stop) and we get:
1024 = s0 + 5v0 or s0 = 1024 - 5v0
which is the point we have to start decelerating.
joey.