A question on 4-20mA precision current measurement
The device you're especially considering lists:


  Load ≤ 300Ω


as requirement. Others give a maximum output voltage (which, in this case, would be 20 mA · 300 Ω = 6 V), yet others actually do list a source conductance in the sensor datasheets.

These numbers typically are the result of both limited supply voltage and limited current drive capabilities within the sensor, and are just as good as giving you a source conductance – you wouldn't use a very small-valued measurement resistor, anyway, because that would make your measurement hard to make precise. 

So, your 250 Ω is a good choice – close enough to the maximum load resistor, and small enough so that it makes sense for your measurement setup.