Hallo Jan,
Your 89 has a basic DC-voltage accuracy of 0,025%_that is really fine.
All what you need is an precisions resistance of i.e. 1Kohm and you have to measure a voltage drop over that if it is serial connected with your (AVO) movement...
An
0,01% resistor costs £ 5-20 typically, but for you-I think- is a
precision of only 0,1% enough god too and their prices of £ 0,1--1,0 are really moderate
Wish success, Karl