M3A offset for 500m zero

[KSC]

I recently obtained an M14 w/M3A to take care of some neighborhood pests we've been dealing with. I got some great holdoffs for a 500m point of aim/point of impact, but, given my living conditions I don't have a 500m range available aside from the two-way variety.

So my question(which I, of course forgot to ask the B4s), using M80 ball(I know), what would my offset be on a 25m short stop or can I obtain close enough results using something 50m or shorter?

I'm not going to be using it for HVT hits or anything like that. I'm pretty confident in my range estimation, which I do using mil-relation. Since most of the times I've ever needed one it's been in the sub 400m range, I'm confident that the M80 rounds will be sufficient.