There are a few different solutions available to solve this problem. Instead of using a simple high impedance divider network, the first possible solution would be to use a stand along op-amp like the OP1177, ADA4505, AD8505, or ADA4091-2 as a buffer between the resistor divider and the reference pin of the in-amp. A second option would be to use a high performance voltage reference like the ADR435, or a reference with a built-in amp like the ADR821/827 to adjust the value as needed. The final solution is to use an AD8295, an in-amp with a high performance free floating op-amp as a buffer for the offset voltage.