Sorry for the delay.
Without seeing the actual circuit it's hard, but I would say try about 7.5k for each of the summing resistors.
You can test things by bringing up a 1k tone and adjusting level so that the right and left meters read zero VU. Then choose summing resistors that make the mono meter read zero. (Or maybe -3. There are different philosophies about this.)
Another way: connect a dual 10k pot so that one section is the left summing resistance and the other section is the right. Set up tone as above and adjust the pot till mono meter reads zero (or minus 3, etc.). Carefully disconnect the pot without bumping the shaft, then measure one of the sections. Choose a pair of fixed resistors that are close to this value. You could fine tune the meter reading downward by adding a third resistor (smaller value, maybe 100-200 ohms, you have to experiment) in series with the ground wire going to the meter.
Another idea, maybe the best: use to 4.7k summing resistors, put a 10k pot or trimmer in series with the ground wire of the meter. Do the tone thing for zero on the stereo meters and adjust the pot for zero (or level desired) on the mono meter.
Whatever you do, check with stereo program (music, dialogue, whatever) to see if the mono meter is reading about the way that you want. Again, mono fold downs can have different levels. A console with a real mono buss might have a mono fader, which the engineer would set as desired. Then the buss could be metered accurately. But without a separate mono buss, you may just "guessing" (by a few db) at what the mono level will be when the stereo mix is folded down to mono.
In the end, resistor values will involve some trial and error. I hope this helps and makes sense!