The lack of help made me do my own research better. Here's what I did. Use a fresh 9V battery to set the drain through a 100 ohm resistor at 9V, while keeping the source and gate at 0V. This gave me a first reading of the Ids at Vg=0 so I could match. Next I used a 220 ohm drain resistor and measured again. Surprise: the Vg=0 matched mosfets matched again (now at roughly -250mV gate). In all cases I measured voltage drop over the drain resistor, which will lead to some errors. My fluke won't measure very small currents, so this was the best I could do for now.
Will they match the same way with 250V? I don't know. Need to build a far better test jig for that. Saturation/performance curves in the data sheets look very flat all the way to 500V, but you know 'paper is patient'.