I would not change the flow rate at all, because that might disturb the instruments internal equilibrium.. that influence is very difficult to estimate, as the analogue circuits are very complex. The instrument has these circuits distributed on top (DC modes customization) and on bottom (AC and A/D), and all work together regarding thermal state.
Also, I really doubt that the internal temperature can be held constant by simply regulating the fan.. especially due to this mentioned distribution of circuits top / bottom.. There's a very delicate temperature gradient between DCV analogue circuits (where "TEMP?" is measured), and the A/D ASIC, which obviously both determine the DCV gain constant CAL? 72. So, it's not only necessary to keep the temperature as constant as possible, but also this gradient..
Another minor aspect is, to keep the internal temperature as low as possible, to preserve the components lifetime (especially these high speed comparators).
If you want to regulate the temperature by the fan, you would have to set the temperature towards these maximum about 70°C internal temperature (55°C ambient).
Don't you have a basement, where the temperature is more constant?
Frank